The simple answer, “because he can.”
The longer answer… whew. Because there are less empathic connections to people you only know over the Internet–you can objectify them more easily. Because given the chance, most people will be cruel to strangers (have you heard of the tests whereby they took a random group of perfectly nice people, and told them, “if you press that button, it gives an electric shock to a test subject in another room”? An actor screamed when the button was pressed; they all pressed it A LOT). Because given the chance, humans will attempt to exercise as much power over their environment as they can. Because humans tend to seek standing, and the easiest way to do this is to climb over those of lower standing. Because the game made it possible to do, thereby giving a veneer of acceptability to it. Because the perception was that the victim could escape anytime, and felt nothing because “it was just a game.”
Sadly, I would guess you’d end up banning a significant part of any playerbase. In a commercial environment, I’d guess as many as 10-25%. Then of course, there’s the issue of catching them.
On Legend we had a vampire who lived in a church in Victorian London. The entry to the church was locked. The entry to the church was hidden. The door to the church automatically closed regularly, so that if it was left open, you wouldn’t have good odds of stumbling across it.
When you stepped thru the door, the vampire cackled, slammed it and locked it shut behind you, flew at you, poisoned you, weakened you, clumsied you, blinded you, and basically kicked your ass.
A favorite sport was leading newbies from the inn to the door and saying, “Go on through, I’m right behind you!”
Reprehensible? Sure. Valid? Erm… well, you could be roleplaying a callous, clever, cynical con man. Lord knows that plenty of players used this argument. Desirable in your game? That’d depend on the expectations of the game. On Legend, we warned and eventually banned people for it. On UO, we tell them it’s their lookout and not to be so trusting. If it happened to me on any Pern MUSH, I’d freak. It comes down to how much policing you want to do, and how much you dislike the behavior, and how much you value the ability to play con men, villains, etc.