I'm no expert, but you know a little common sense tells you how people snap.
- They feel invisible. Bombings and random shootings are a cry for recognition that they (the perpetrators) exist.
- For the thrill. Modern society has become so satiated with instant gratification and lack of discipline that many adults have absolutely no impulse control.
- No ethics or no faith. A follow up to lack of discipline, but consequences for your actions is a strong deterrent. Not just fear of jail or execution. Consequences after death are much stronger on people who don't have much to live for in this one.
- Lack of manners. Rude people have little respect for those around them and if the people frustrate them beyond a certain point they will lash out indescriminately.
- Sociopaths. Physical or economic bullies. Those who don't feel (no matter how horrible the crime) that they've done anything wrong and that any punishment is undeserved. Sound like a few banksters and hedgefund CEO's. This works both ways the sociopaths make become violent if they don't get their way, but if they pick on everyone else too much they may become the target of their prey.
- Lack of justice. Those who feel that they've been wronged and society, the police, the laws, the politicians can't, won't do anything about the wrong, or are even enforcing the wrong. It is at this point that organized crime gets started. When the government fails in its duty to protect law abiding citizens, then those citizens will turn to criminals to seek vengeance. Watch the opening scene in the Godfather. The man who's daugher was beaten and raped wanting justice because the two men who got off, best example.
- Extreme poverty. There always come a point at which the poor snap when the gulf between them and the ruling 1% gets too big. See French Revolution. Russian Revolution, American Revolution (don't think for a minute that it wasn't about Parliaments preference of monopolies at the impoverishment of the colonies).