I agree 100% on people with mental illnesses being held responsible for their actions. I don't know a lot about intricate details of mental illness and how those people function (or don't function I guess, depending on how you look at it).
My question is where/how do we start? Even if all the funding in the world were available, with 100% backing by every state, mental institutions readily available etc...where would you even BEGIN to identify people who are "at risk" to commit a violent crime? Some of these people are what we would consider " a little off" or "very passionate" about specific issues, but is it enough to lock them up under the fear that it MAY HAPPEN? And who is going to contact authorities and tell them "hey...come get so and so...he's crazy" and they actually KEEP them there...for, what, life? Because if they are locked up for an amount of time who's to say they aren't building up the angst and plotting revenge every day they are in there? The odds of someone being held for life is slim to none.
And when these people plan on offing themselves when they commit these crimes there is no way to win this war. I refuse, 100% refuse to give all of these killers the label of mentally insane or mentally ill. I may be short sighted, ignorant, etc. but I believe that what we see a majority of the time is fueled by nothing less than pure revenge. It's their own self pity taking form in the most coward-like manner possible. People aren't necessarily crazy just because they kill someone - sheer hate can fuel the desire to kill, and until we are able to accept that, I'm not sure where the heck to even start to prevent something like this from happening.