Behaviours of Characters/People with Psychological Problems

I was reading another manga from an author I like and as always I like to check the comments right after I finish it. And there’s one particular thing I find interesting about the comments on this author’s mangas; There’s always a comment that says something like: “I’ve read many twisted and fucked up manga but I always fail to understand the thought process, behaviours or the motivations behind this author’s characters.” And then some thoughts struck me:
What exactly is the behaviour of a person with psychological issues/disorders?
Is it really much different from a normal person?
Does a mentally ill person need to have certain motivations behind their actions?
What type of characteristics in a mentally ill person do you usually see when you read a manga/manhwa/manhua?
Is it hard to understand a mentally ill person?

I examined these questions and racked my brains to try and figure out what the difference could possibly be between this author’s characters and what you usually see.
Could it be that this author had grasped the true form of these behaviours that the other authors just had a misconception of?
Or is this author the one with the misconceptions?
Maybe people aren’t used to seeing the type of characters that this author writes, hence they don’t understand them?

To get an answer to the last three questions I thought I needed to understand the former few but if you have any other ideas to answer them then please do share, however little it may be. ლ(´ڡ`ლ)