Yeah, this was along the lines of what I was thinking. A lot of mental illnesses (disclaimer: I do not have one, I'm just speculating) seem like over-manifestations of normal and necessary psychological processes.

Having a clear sense of 'use vs them' and being aware of one's social standing are natural parts of our psychology. People do steal, lie, backstab, and spy to gain advantages in life and having a sense of when somebody is 'after' you or out to get you is important to avoid being endlessly exploited.

A close childhood friend of mind developed paranoid schizophrenia and my memories of his paranoia blur from 'normal shit teenagers worry about' to clearly pathological delusions, with no sharp line in the middle. It was almost just like normal paranoia turned up to 11, then it just kept going to the point of complete delusion. Even when it started to get noticeably bad (neighbors taping him kind of stuff), there was something believable about most of his stories. His brain clearly wasn't stupid or broken, it was just... overconnecting dots.

I think humans evolved some balance of ability to deceive vs paranoia, and I think those mechanisms can go off the rails. So maybe it's not that we evolved anything specific to enable schizophrenia, but that we evolved the brain circuitry for 'paranoia' along with a way to moderate it, and something causes it to be incorrectly moderated. I'm sure real life diseases are much more complicated than that, just a shower thought. Of course there's also the whole component of hallucinations too and all that.