Everybody is different, and you never know how anyone is going to react to a specific medication. Millions of people are on meds for psychiatric diagnoses, but very very few become mass murderers because of them...similar to millions of people having legal firearms (me included), yet very very few of us become mass murderers like this a-hole.
The mind is complex...could she have put him on a med that contributed to his behavior? Yep...but could she have known? Nope...