Well..we're studying serial killers in my criminology class right now (that class rocks) and it's actually very interesting. Their reasons for killing usually come from their hate/love relationship of their mother. They feel that she controls to much of their life but because they can't sever the ties to her they kill other people as like..stand ins. Also, out of 30 serial killers this one study performed a test on (all were male), all of them had an extra Y chromosome. That means they're an XYY, and that extra Y chromosome means extra male hormones which would make them more aggressive. This also means that they may not be completly sexually developed, meaning they can't have sex. They sometimes relieve they're sexual frustration in the form of killing victims. Also, serial killers are known to have repressed anger. They kill because they let their anger get pent up inside, and when it gets to much they explode and kill. Then they calm down, and don't kill again until the anger becomes too much. Of course..it's still unclear if this sniper guy is a serial killer or not.