I grew up with the band Korn. In junior high, I got their Deuce DVD, which is basically a compilation of footage of the band members fucking around. I remember being shocked at how happy and goofy they were. After a few years of listening to their incredibly dark lyrics and music, seeing them cracking jokes and laughing for 2 hours really weirded me out. Their personalities did not match the music they played, and that bothered me. It made Korn seem...I dunno. Insincere. I was expecting them to be mopey, depressed goth guys. When that turned out to be the opposite of what they were really like, it did disappoint me a little.
Funny thing is, this has happened with a number of rock stars over the decades. Back in the mid seventies, kids were shocked the first time they saw Alice Copper on tv acting and dressed like a normal dude, not the monster he portrayed on stage. I guess that's just your perceptions as a kid; you really want to believe that your idols are these Gods on some Olympus of Rock, and not just normal, everyday people trying to pay the bills in their own way, a way which happens to be highly entertaining. I guess most just grow out of it eventually and accept rock stars as normal people (except for those religious crazies that really do believe Ozzy Osbourne and Dio are agents of Satan).
As for the topic of this thread, there are a lot of artists I thoroughly enjoy who's perspective on life I completely disagree with, but as for people who's personalities I find really irritating, even if I do enjoy their music............... the top spot easily goes to Geoff Tate. Between his public comments, his blatant attempts to re-write his own history, and the way he has run his band into the ground over the past decade and a half, I find the man intolerable. Alexi Laiho comes in as a really close second, though. His inability to just keep his mouth shut about other bands really pisses me off. Zack Wilde is pretty bad, too. Great Guitarist, but a shitty attitude. Andrew Eldritch is up there as well; it's mostly the fault of his ego that Sisters of Mercy can't maintain a stable lineup. Oh, and of course, Lars Ulrich.
Conversely, I can think of a number of artists who have a reputation for being total d-bags, but who based off interviews and other media, really don't seem all that bad in comparison to some of the people I just mentioned. Yngwie Malmsteen and Blackie Lawless in particular seem to have had their reputations blown waaaaaaaaaaaaaay out of propotion. I really don't think Mike Portnoy is all that bad, either; not by a long shot. He doesn't seem like that bad of a person, he just needs to 1) learn when not to say anything, and 2) be a bit less of an OCD control-freak.