Regarding your first point - usually because the 'dark side' is easier (short term), and life periodically gets hard. So it's often inevitable. Especially in the face of others doing bad acts to you.

Regarding the second - see my comment.

At least in the western world, most crimes require fault and mens rea. Those require someone being able to make a informed choice, which means having free will. If there is (actually) no choice, or the choice was based on faulty information processing (aka actual insanity), then therefore there is no fault and no mens rea.

The underlying belief (philosophically) is one cannot be good or evil if one cannot comprehend a choice or it's ramifications, or in fact actually effectively choose between options due to a lack of knowledge and independent agency.

Interestingly, this is why the 'original sin' in Christian mythology was eating the apple from the tree of the knowledge of good and evil. [https://link.springer.com/referenceworkentry/10.1007/978-0-3....]

So philosophically, before that point, there was no 'good' or 'evil' because humans didn't know of such things, therefore were just acting. Regardless of what they did, they were innocent, because they were incapable of being anything else. After that, everything they did had such knowledge as a factor, and were no longer innocent - because they could choose.

So, for example, if I murder someone in cold blood (aka rational/thinking/choosing), that is 1st degree murder. One of the most serious crimes in any society.

However, if I murder my spouse because I found they betrayed me right in front of me and lose my mind (literally can't think straight, and react instinctively without being able to think or plan anything), most locations will consider me not guilty by reason of insanity, and let me go. [https://www.law.cornell.edu/wex/temporary_insanity]

This is already a great example of the problem, as MANY folks try to claim insanity already after getting caught when they are not actually insane.

Another example is if I accidentally kill someone without knowing such a thing was a possibility. For example, I'm out hunting in the woods. I see a deer, and (as far as I know) legally shoot it. However, it turns out that was actually a human dressed in a very convincing deer costume, and I had no way of knowing this was the case. Most of the time, I would be charged with no crime, or worst case likely negligent homicide.

As per the Supreme Court of Iowa, "In order to be an excuse and defense for a criminal act, the person accused, and who claims [temporary] insanity as a defense, must prove that the crime charged was caused by mental disease or unsoundness which dethroned, overcame, or swayed her reason and judgment with respect to that act, which destroyed her power rationally to comprehend the nature and consequences of that act.”

If we assert that people actually act based on their nature/upbringing and have little to no rational control over their actions in fact (aka no free will), then does it not follow that no one actually has a choice, and therefore it makes no sense to actually punish anyone for anything they do? Since philosophically, without the ability to rationally actually choose their actions, they cannot be good or bad - they just are.

And if we punish people based on who they 'just are', that opens another can of very nasty worms, and likely a lot of serious evil will result.