From what I have read, proper dosage is critical, as is the case with any drug. Just enough, and the person feels better. Too much, and bad stuff happens.
Some pharmaceutical companies have been successfully sued over this in recent years, although it doesn't get a lot of press.
Here is a larger list of anti-depressant related violence:
http://www.ssristories.com/index.php
And no, we don’t know that he was prescribed Prozac or that he was even on any medication. That is an unconfirmed report from the Chicago Sun Times. But I’d put odds on him being on some sort of anti-depressant.