This is often the case. More generally, every crook has a sob story. And it's important to not let that cloud your judgment.
In judging a harmful person's character, all that matters is whether that person has definite potential to become a net-good person given spiritual instruction, social support, medication, or whatever. But if such a process is infeasible, then the person needs to be written off. Some people are irredeemably lost, and the only thing you can do is love them and pray for them, at a great distance.
This is a good time to employ some Occam's Razor: if we've already accounted for the shittiness via mental illness, what is our reason for introducing an additional explanation? Are certain extreme mental illnesses not enough to account for the most shitty of behaviors?
ok, so this is more like the Linda Problem. It is more likely that a person is a shitty person than a person is a shitty person and has a mental illness.
Hmm, there's an element you've left out of the situation that makes it different from a straight Linda Problem though. In logical terms, it looks like this.
And we know know two things: 1) the person in question is exhibiting shittiness 2) the person in question has been diagnosed with a mental illness. So:
mental_illness = true
shittiness = true
Whether being_a_naturally_shitty_person is true or false is something we have no information on.
So, from that starting point, we already have an account for the shittiness being exhibited since:
mental_illness -> shittiness
and
mental_ilness = true
So if we want to go ahead and make the additional claim that being_a_naturally_shitty_person = true —you're violating Occam's Razor by introducing an unnecessary second cause when we already had a sufficient first one.
I suppose the answer to that depends on how meta you want go get, so to speak. But at its most basic, some people are just assholes without being irrational, stressed out, etc. Though you could easily argue they weren't raised right and are being stupid, basically.
I'd argue that all negative behavioral issues are mental illnesses. One way to classify it would be biological mental illnesses and learned mental illnesses. For example autism being a biological one and PTSD being a leaned mental illness.
Any time you have multiple people interacting, you will have some friction, even in a hypothetical situation where all involved parties have perfect mental health. Many times, behaviors are classified as "negative" based on a judgemental third party perspective.
I raised two special needs sons. They are okay in part because I wondered why they did things. I was curious about the first person perspective. I was interested in finding out "What's their motivation?"
So I think this is a much more complicated thing than you are making it out to be. People have a relationship to themselves. They have a relationship to their environment. They have a relationship to other specific individuals and to a larger society.
Mental health discussions often posit that you are mentally healthy if and only if you get along smoothly with other people. They often don't wonder at those other elements and they don't question the possibility that maybe it is the other people who are all messed up, not you.
I think this is a vastly complicated topic. It is both broad and deep. It touches on psychology, sociology and philosophy.
And I don't know that HN is really a suitable place to try to dig into it in a meaty way, frankly.
Also, sometimes you can have empathy, but can't personally handle someone. You have to do what's best for yourself, also.