A friend was riding with a small group and there was an altercation and they flipped the driver off. Driver went around the block and came up behind them and knocked my friend into a parked car, then sped off. My friend was pretty banged up but nothing serious.
Another friend was alone and a driver followed him (after an altercation) and knocked him into a parked car on his own street. He was unconscious for several minutes.
Another friend was run off the road by a commuter. He's certain it was intentional but has no "proof". I believe him.
No matter what your view of humanity is you have to admit there are some seriously deranged individuals in the world. When you encounter thousands upon thousands of drivers over the years you are bound to encounter a homicidal maniac or two.