if one were to have a mental illness that causes them to be very impulsive such as they engage in zina/drugs/etc... are they charged by Allah the same as one who commits these acts but does not have a mental illness that sometimes impairs their judgement or that makes abstaining from these acts difficult? i read somewhere that one accrues a bigger sin if it is easy for them to abstain but choose not to than one who it is difficult for them to abstain.
The Quran says:
God does not burden any soul beyond its capacity (The Quran, verse 2:286.)
We do not burden any soul beyond its capacity (The Quran, verse 6:152.)
We never burden any soul beyond its capacity (The Quran, verse 7:42.)
God never burdens a soul beyond what He has given it. (The Quran, verse 65:7.)
Each person is judged according to their own abilities and weaknesses. A sin that is easy to avoid for one person may be very difficult to avoid for another. The same applies to good deeds. A very rich person who gives away $1000 in charity is very different from someone who only has a little money saved and gives away $1000. The second person is making a much greater sacrifice and will be rewarded differently.
Most sins and good deeds are partly because of human nature, partly because of free will. If a person is almost overwhelmed by human nature to commit a sin so that there is little room left for free will (such as a person who takes drugs for Parkinsons’ disease, which can cause compulsive behaviors like gambling), then their responsibility will be much less than another person who chooses to sin when they can easily avoid it.