To me, glorifying someone like him by wearing his picture on a T-shirt, poster avatar, whatever, is part of what helps erode our sense of right and wrong.
From my viewpoint, it's the little stuff that helps foster a certain apathy. That's like the kid who wore a T-shirt with a swastika to school. It's only a shirt, right?
Certain acts, though small, add up to helping people have a skewed sense fo what's appropriate, or "right". It's the day to day stuff of letting things pass like that, that I believe desensitizes people to what's appropriate. It lays the foundation to more extreme things being "okay", cause it's just a billboard, or TV show or whatever.
I think an avatar of Jeffrey Dahmer would be wrong. So would Hitler. a kid looks at that and says, well, that's must be okay, he's doing it.
I'm not saying, nor did I say anything as ridiculous as "Gee, that guy's in an avatar! Killing people must be COOL!", but I think that kind of stuff lends itself to making some people think that's acceptable behavior and I think that helps contribute to an off kilter sense of right and wrong.
Just my opinion.