One of the arguments which are good to prove the free will is something we all experience and that is what we call regret. I do something and later on I think that it wasn't appropriate. I speak about something and take a particular action towards a particular case. For example I get heated or lose control and afterwards I feel I wish I hadn't gone that far, I wish I could control my anger, I wish that somebody reminded me not to go that far.
It is something psychologically known by all. For example when Imam Ali (a.s) heard somebody saying Astagfir Allah, I seek forgiveness from Allah. The Imam said to him "Are you fully aware of what repentance means? Or are you just saying something without concentrating on what you're saying?" Then the Imam starts explaining that there are six steps, six requirements, and six elements to be secured before knowing that this is real repentance. The first one he said ( الندم على ما مضى ) the first step towards sincere and real repentance which there are hundreds of ayat in the Quran and the Ahadeeth of the Holy Prophet (p.b.u.h.) Ahadeeth from the error free Imams about the significance of Tawbah, repentance. So the very first step according to Imam Ali (a.s.) is to regret what has happened because if it was the other scenario that somebody did something wrong, did injustice or backbiting or anything like that and now wants to repent and inside he says "it was good of me that I did that!" This is not repentance. The first step to repentance is that I really regret what happened.
And now this regret is a sign of free will because if I was tied up, if I had no choice, if it was written on me and I couldn't change it so what does it means? What does the role of regret present to us here? It is illogical. Why do I regret because something is for example if my pulse or the way that the oxygen is inhaled in my lungs is less or more, I can't regret it because it is not made by my own will. So if something is beyond my control, beyond my will, why should I regret it? So regret is a big sign that I could avoid it, in reality I was able not to do that particular thing and since I did it I regret that.
In the Quran we read that when the sinful people on the day of judgement are facing the consequences of their wrongdoings they say (قَالَ رَبِّ ارْجِعُونِ. لَعَلِّي أَعْمَلُ صَالِحًا فِيمَا تَرَكْتُ ) Oh Allah give me another chance to go back and I might do something good. And Allah responds to that (كَلَّا ۚ إِنَّهَا كَلِمَةٌ هُوَ قَائِلُهَا) because many chances were given to him and he's only trying to express what he faces the chastisement, when he sees with his own eyes that how he's going to feel the fire of hell and all those calamities and problems which are the results of his own disobedience and bad behaviour and he says "Oh Allah if I go back I promise to be good" which means that now he feels that he was wrong otherwise he could easily say "Oh Allah, it wasn't me, it was You who forced me to do it." And this is one of the essential arguments about the free will.
So if we are going to conclude we see that those who deny the free will, in reality they are trying to refer or attributes of justice to Allah, whilst Allah says that (وَمَا رَبُّكَ بِظَلَّامٍ لِّلْعَبِيدِ ).
Those who try to stick to Allah's will or Allah's knowledge to make excuse that they do not have free will, in reality they are talking something illogical, and in the end let's say that those who were extreme in negating the free will in opposite to that there were people who said that everything is left to our choice and God has no role in our decisions at all. But when Imam Jaafar Assadiq (a.s.) was asked about that, he clearly said (لاجبر ولا تفويض بل أمرٌ بين أمرين) It is neither predestination in a sense that we lack every free will, we can't choose anything and it is not the other way round that we are left openhanded and God does not intervene in our work but it is something in between. And by in between, the Imams and specially Imam Sadiq (a.s.) try to make it clear that our being, our limbs, our organs, our ability to think, all the tools which end up with doing something are from Allah. But how to use them in this way or that way it is from us. So in this way whatever good we do, we thank Allah for enabling us to do that good, if we pray, if we pay zakat, if we fast, if we go for pilgrimage, if we do charity, and if we care for others first we thank Allah for enabling us to do all these, for providing us with the tools which allow us to do that good work.
But when we do something wrong, Allah says that "I provided you with this but you misused it." So having wealth or access to money for itself is good, if you use it in whatever helps or promotes education, knowledge or has a role in giving good results to humanity, discovering scientific things which helps promotion and elevating knowledge, this is one way. And the other way is that somebody uses that money in gambling and spending it on their lust and ill desires. The money is the same money, the wealth is the same but the way it is used is something which we choose and that is the whole concept of Adil or justice which is the second pillar of our faith.
One of the subjects which are related to divine justice is the problem of evil or what is known in the books of theology as theodicy. The whole issue is this, that we see earthquakes, floods, tsunamis, natural disasters and many similar things. In the West they call them acts of God but let's deal with them in its appropriate way, it is called evil (مشكلة الشرور) and the question is that we believe in Allah being all powerful, omnipotent and all merciful. So definitely we see these natural disasters which cause a lot of damage to humanity, to nature and hundreds or thousands of souls are affected by them so the question is that either Allah is able to stop these but He doesn't which means that He's not merciful or He is merciful but He cannot stop them so by definition you said that He is powerful. So being powerful on one hand and being merciful on the other hand having both these together means that He has to stop all these natural disasters and doesn't allow them to have that impact on people and on nature.
In theology we have various responses to this question, first of all let's see that is evil something real or when the absence of something good is there then we call it evil? So it is our expectation and in this way let's differentiate between two things, first we are talking about the clash of interest. Let's assume that there is farmer who is cultivating the land, the seeds are ready, everything is there, he's waiting for a good rain to come and irrigate his land which is needed for the growth of the herbs or the vegetables. Next to him another person who has recently finished building his house which is mainly made by clay, so the person who is waiting for the sunshine to let his house dry up and get his strength he's asking Allah "Oh Allah, don't send rain."
The farmer on the other hand is ready and is waiting for the rain to pour and irrigate his land is saying "Oh Allah, I'm begging you please send me some rain." So when there is a clash of interest here and there, do we expect Allah to listen to one and not the other? Or when people think something is matching their interest then it is good and if it is not matching their interest they consider it evil?
In reality we are all like that, when everything is good we call it good luck, when everything matches our expectations, when it comes in our own line and serves our purpose we consider it good. When it is against our expectations we call it evil. This is selfishness of people, this is one thing and the second thing is that are we talking about the balance between everything and each one relatively is good for certain purpose which is not good for some other purpose? Let's see the difference between the jumbo jet which is able to carry 450 passengers and with a fighter plane which is used for military. The passenger carrier airplane can carry 450 passengers but doesn't go as quick as the fighter jet. The fighter jet is many times quicker in comparison to the jumbo jet but can only carry two people. So to expect one thing too be able to carry 450 passengers and at the same time to go as fast as the fighter jet is wrong. Each one is good for itself.
We have the race horses and we have horses that carry loads. Sometimes insurance companies are ready to insure the race horse for a million pounds but they are not ready to insure the horse which carries the loads for even ten pounds. Why? Because the horse race is good for itself, serves a purpose which is totally different from the other one. So to expect that all horses must be having the same quality and the same ability is wrong.
The other point which we raise here is that when a lesser degree of harm is acceptable to avoid the greater so the lesser of two evils is logically acceptable everywhere. For example when somebody is affected with gangrene in his foot and the doctors say that if we don't severe this part of the foot or the fingernail this will grow and spread and in the end we have to cut the leg from the knee.
So always locking those who are dangerous to the society and depriving them from their natural rights is more important in comparison to the bloodshed or corruption they cause in the society.
Let's conclude that we cannot judge everything that we see and think that it is evil. We cannot consider that as a general ruling or case or argument and say "why isn't Allah able to remove these things?" sometimes there are evil things which are for our good and for our goodness and the last word we shouldn't forget that many of these disasters are the direct result of the human interference. Let's assume that those who throw the nuclear waste in the bottom of the oceans, if they cause some sort of fraction or something in the bottom of the ocean and this causes a tsunami or something like that, who's going to be blamed? If we do not take the necessary precautionary measures to avoid any spread of illness, who is responsible for that? If I do not take the proper exercise to keep healthy and in the end I lose my ability to deal and cope with my life, who is going to be held accountable for that?
So the conclusion is what is called the problem of evil we can't refer it to Allah. First it relative, second is clash of interest, third is the human interference in the whole nature system and when everything is analyzed for itself we see that this question is not the right one and can be answered easily.