i've seen intentional visciousness. when i see it i try and think:

hurting people will lash out.

it is a fairly primitive response. if you put a couple of monkeys in a cage and you give one of them an electric shock then that monkey will lash out at / physically assault the other monkey even though it is clear to that first monkey that the second monkey was not the cause of their pain.

hence, given that people with mental illness are often in great pain it is probably more likely that people with mental illness will lash out at others at times.

there is another point with respect to intent.

when people have had experience of others being abusive / sadistic towards them then quite often that person comes to believe that the world is a fairly unsafe place and that people in general have abusive / sadistic qualities. that means that one is more likely to interpret the actions / words of others in such a way that they are read as having abusive / sadistic intent. while happy healthy people find it fairly easy to assume the best of others and interpret their actions / words positively and assess hurtful / hateful words as being a sign of pain rather than genuine threat people who haven't been so lucky tend to find that much much harder.

so... while it would be nice to think that experience with mental illness would result in people being less judgemental and hurtful and hateful and abusive the sad fact is that people with mental illness (given their experiences) are probably likely to be more judgemental and hurtful and hateful and abusive than most.

of course it is possible to turn that around... but it takes considerable time and effort and during times of stress / distress we tend to revert back to what is familiar.