That’s because the mental health excuse is just that: an excuse. They don’t actually have the evidence to back up the notion that she’s mentally ill other than her diary entries. Those entries could have been forged for all they know.
They not only undermine the very real damage that woman caused by using mental health as a cynical attempt to try to give her an out, they also are being extremely ableist. Committing egregious crimes != mental illness and for them to draw that equivalency caters to the stereotype that mentally ill people are dangerous.
These are people who know that and who would call others out for being ableist, yet do so freely in threads like this without consequence or a second’s thought from anyone else. Ask yourself why that is.