As many of said I think its an element of the younger portion of the game's players doing this. Its a naive assumption that death is the worst thing that can happen to anyone. Call this dark thought if you want but in comparison to being killed, which in respect to other things is pretty abrupt, short, and ended compared to other types of prolonged suffering. Most of my characters, if they threaten harm at all, do not threaten with death/killing.
What I find rather amusing and contradictory is that usually the SAME people who have characters who make characters who threaten killing are the same ones who make chars who are NEVER afraid of being killed themselves. If you're REALLY trying to make it seem like being killed is THAT bad and the absolute WORST thing that could happen, shouldn't that mean your character KNOWS how bad it is and if someone parades up to them with the same threat that they should be concerned as well instead of growling/snarling/"being ferocious/etc? Oh wait, scratch that. I forgot, nearly everyone has "super-fearless-overpowered-Mary Sues that are never-ever scared of anything". Nevermind. >_>;
But again, really, getting killed is'nt the ONLY bad thing that could happen to somebody. You'd think with the number of "my parents abused me and sob sob sob cry" characters that being beaten to excess or being maimed would be an equally common alternative to saying "kill" all the time, but its not. So is that to say, according to that logic, that your sob-story char doesn't have it as badly as he/she claims to? Oh wait, "Oh but you don't know what its like to be abused and blah blah blah" Obviously these folks don't know either, as their logic massive holes in it.
Flipping the "kill switch" on your character when things don't go the way you [or your character] would like is'nt a proper solution to keep a RP going, not even for an evil character. And its not a good thing to "flip the switch" JUST because you're bored and want to create conflict. There is more than one avenue to some kind of fight or conflict without killing being involved. I'll give a little example,
Mikey is one of my evil characters, and lets say he's looking for Character A for some reason. If he can smell Character A is in the area but isnt anymore and believes Character B might know something about where Character A went, he is going to ask. Afterall, he just wants to know and move on to finding Character A. He doesn't care about Character B. And if for some reason Character B tries to withold that information and Mikey doesn't get the answer he wants, he's not going to kill Character B. He's just going to threaten to torture them and if they refuse to answer again, do so until he gets the information he wants. That MIGHT put Character B on the brink of death but not necessarily, especially if Character B gives up the answer. He doesnt barge in and go "IM GOING TO KILL YOU IF YOU DONT DO WHAT I TELL YOU TO." That does'nt make a character look intimidating, only barbaric and sometimes, stupid.