It's the thrill of the kill. Trophies are okay, but that momentary rush attained from besting an opponent is what it's all about.
People with high Killer scores prefer the player-versus-player aspect of any game more than anything offered by the environment. They relish the adrenaline and challenge of pitting themselves against real people. Not all Killers are gankers as some critics claim. While getting the jump on the unprepared for a quick kill is important fun for some Killers, many prefer group PvP and revel in a well executed team maneuver that blows away the opposing side.