If your friend says something like "I want to kill people at school" your response needs to be "uh... that is not cool'' or something of that nature. Not, "dude, yea". This kid gave all out all the signs. He had even told people at school what he was planning but nothing was done. Watch his Flash animation if you don't think he sent out warning signs.School Killer's Animated Terror - March 23, 2005
The kids who did all the damage at Columbine had created a Doom .wad file of the school. They were hoping to use it to practice running around shooting their classmates. I was able to find it and download it before AOL took down their websites the day of the attack. One website of theirs had nothing but "Your momma" jokes. The .wad file wasn't that good but I bet if they had Flash back then, they would have created something like this kid.