First of all. He is basically a kid who chose his AIM screenname at age 14 to be something that he thought was really cool and dramatic (kreetureofnight1991!) and now it's 50 years later and he's still stuck with it, because he already has his buddy list and it'd be way too much trouble to change it, isn't he? I mean, Lord Voldemort? Because it is an anagram? I bet when his Death Eaters call him that, he is all, *facepalm* but he pretends he doesn't care and that he still thinks it's cool and not at all embarrassing. Either that, or he and Lucius Malfoy sit around drinking beer and being all, "God, do you remember when I thought 'Voldemort' was a cool name? Man, we were dorks! Oh well, let's go torture some Muggles."
QWP, of course.