I think a lot of it has to do with the way we view things in general. We used to see the world in black and white, either something was good or evil. Now we see a million shades of gray. There are 'good' people who do bad things, 'bad' people who do good things, and neutral people who don't do much of either.
We've also weakened the idea of 'evil' through overuse. There are vegetarians who call meat-eaters evil, kids who call their teachers evil, and the amount of people I've heard being compared to Hitler and Nazis is frankly mind-boggling. I heard someone get called a Nazi for telling someone they couldn't wear roller-blades into the movie theatre.
Society's ideals of good and evil have been so watered down that they don't really mean anything anymore. Even those who commit heinous acts are partially forgiven (and sometimes idolized). They blame their circumstances, their parents, their psychological problems, violence in the media, and just about anything else. Because of this, we've all become convinced that nothing is truly as evil or as good as it seems.
And the point about body image really is very good. The "Devil comes in a tempting form" (I'm paraphrasing, I don't have the exact quote but you get the drift). People like things that are pretty. No one likes the old-school hideous vampires or zombies ('likes' meaning, for example, wanting to be one). We prefer our undead (and regular) evil as attractive as possible.
And under no circumstance is killing a cow as bad as murdering a human being. That's just idiotic. If we're going to put animals at the same level as humans, we might as well add insects and plants to the mix too. the comparison is about equal. The fact that any thinking person could believe that, actually hurts my brain.
