You're very right. I still don't understand what pleasure they get from being sly or nasty- but they must get something out of it. I know I'd feel bad if I ever hurt someones feelings or deliberely destroyed someones life- but some people just don't have that conscience.
Usually they are attention seekers or desperate social climbers that don't want to put in the hard yards or find it hard to appreciate someones efforts if it's not there own so they have to interfere or sabotage. There are plenty of them in this world.
Edit: Murderers are a whole other ball game, however they probably start off thinking everyone around them has it easy. They should know there are people dieing of cancer that are just happy to see tomorrow.