The line is usually drawn by the courts, doctors, psychiatrists etc
Not all arseholes have mental health issues, but I'll stick my neck out and say an arsehole who stabs multiple young children is highly likely to be seriously unwell, mentally.
I don't think anyone is arguing that tbh.
I don't think it's a stretch to say that while it wouldn't completely eradicate incidents or individuals like this, more and better funded services should identify and mitigate.