Delta9, you flat nailed it with that one. That was the exact point I was driving at. A couple of my closest friends have bipolar, and they're wonderful, intelligent human beings who care more for people than they know what to do with. They would never harm themselves or anyone else, either on or off their medication. Anecdotal evidence, I know, but personal experience really informs the way we think about issues like this.
By the way, in efficacy trials for bipolar, antidepressants alone perform incredibly poorly. Typically, a mood stabilizer (such as Lithium Bicarbonate) is required to get people back on good footing, and then other drugs are added to the mix to keep people in ship-shape. The drugs aren't typically sedatives that numb the patient, but rather are a means for sufferers of bipolar to get back to normal. I don't care what anyone says, when someone is so depressed that they can't pry themselves from their beds for days or weeks at a time, or when they spend hundreds or thousands of dollars on frivolous things in a matter of days, they are not normal. When properly treated and medicated, bipolar patients get out of bed and go about their days like anyone else, and they don't have the impulses to behave in rash and silly ways.
I guess my point is that while this monster might have been bipolar, it wasn't his mental illness that made him a monster; his extreme maliciousness and selfishness did. For that matter, it wasn't a gun that made him a monster either; the fact that he willingly chose to use it to take the lives of others did.