I was married over 30 years before divorcing my ex. Homicide never crossed my mind as a viable strategy for ending the marriage. I don’t understand people who think that way, although I suspect that there isn’t a lot of thought involved.
I don’t think how they met is relevant. You can meet seriously unbalanced people online, but then you can meet seriously unbalaced people at church, at the bar, at the post office, at the grocery store, etc.
Meeting alone with an ex is probably a bad idea, in general.
As far as why it took them so long to file charges, they had to be sure they had darned good evidence to counter the “self defense” claims. Unfortunately for her, “the devil made me do it” isn’t a good defense strategy. (Neither is the old Southern classic, “he needed killin’.”)
30 years married?? 25 years here, and despite unimaginable difficulties, financial and mental illness, I would NEVER leave my precious wife.