If only there was a simple answer for that perhaps we could prevent such tragedies. I am not sure that we can do anything to stop such behaviors, but we can be more involved and aware. I have to believe that most people who do things like this do give warning signs. The Virginia Tech guy is a primary example. Had someone intervened earlier and gotten him the help he needed, perhaps it wouldn't have evolved into that tragedy. A lot of people noticed things were not right with him, yet nothing was ever done. I also believe that mental illness does carry a stigma and it prevents people from seeking help. Perhaps they fear they will be seen as weak or powerless or something. I don't know... I doubt anyone does.
I personally would like to see people stop being so afraid of each other and paranoid. This leads us as a society to become too self-involved and less empathetic with others. When we lose empathy we often don't see what's right in front of our faces.
As for me personally, I will continue to live happily, love and help others, and hope that i never encounter such an individual - but I refuse to be overtaken by hatred and terror as people like this intend to inflict on society.