What do you get out of killing other players when they are just minding their business?

I am new to the game. I stick to myself for now. But I've already lost count of the number of times other players go out of their way to kill me for no apparent reason.

Today I was just riding down the street in Valentine, minding my own business, and some bitch player came up behind me and shot me in the back. WTF!? Why? What do you get out of doing that? Are you just an asshole? Is that it?