This is just my opinion, and I’m not disputing what you’re saying at all. I think we probably are mostly in agreement, just responding since you brought it up.
I don’t think it’s so much making excuses for mental illness, but accepting that it’s a factor in situations like this while still finding the crime unacceptable. I wouldn’t say that it’s making an excuse, but acknowledging it can help us make sense of why this happened and what needs to change to prevent it happening again.
As you say, this person was free to roam while people knew of her mental state. If it turns out that the services that were supposed to help this person dropped the ball multiple times along the way, unlike the perpetrator, none of the people responsible for helping her will see consequences. And I have seen this in other cases before. When the government services that are supposed to prevent this are not fit for purpose, the only person blamed is the perpetrator. Of course only one person committed the offence, but that doesn’t matter - when situations like this happen we all pay the price of inadequate support systems.
And yes, lots of people with mental illness or troubled pasts live their lives without hurting anyone ever. But that seems like a moot point when the reality is that a child is dead and there’s a possibility that we as a society could’ve done more to prevent it.