We identify with murderers.
Everyone, regardless of how well-adjusted they are, at some point in their lives, will fantasize about killing someone. It's a perfectly normal reaction to a stressful situation. Whether it's your dickhead of a boss, that jackass motorcyclist who thinks he owns the road, or that who you can't even think of without visualizing them in an elaborate Saw-esque torture scene--though we may never act upon these thoughts (us being perfectly sane individuals who can supposedly control our own actions), we have them because they make us feel good in some way. Murderers simply acted on these thoughts. Though their actions are certainly inexcusable, we can at least understand them to some degree.
Now take child molesters.
We can't understand these people. Nobody in their right mind fantasizes about molesting a child, and most people who do hate themselves for it, let alone others. Put a child molester in a room full of murderers and pretty much everyone is going to agree that he is the most evil person in the room--sometimes this includes the molester himself. People who might be against the death penalty for murderers may have no problem publicly hanging a child molester (which goes back to what I said before about homicide fantasizing).
And then there are animal abusers.
You ever see those hippies who are all "peace" and "love" and "pot" and seem like pretty chill guys, and then they go all Super-PETA when they hear about an animal abuser? Tell them a kid got killed and they'll be all, "wow, that sucks," and then they'll move on; tell them about a puppy that got abused/killed and they'll come up with all sorts of elaborate punishments that they believe the abuser should be subjected to.
Again, it all comes down to understanding. We can't understand them because we can't understand their thinking; whether because they're afflicted with some disease or just sickos, us "normals" can't understand them because we can't think like them. We are completely incapable of understanding the mindset of an animal abuser, or a child molester, or a wife-beater. But we can to some degree understand the thought process of a murderer. And once we understand, we automatically empathize. That's why murderers don't seem to get as muck flak as child molesters or animal abusers, even inside prison.
None of this is to say that murders are justifiable, of course. Only that they are understandable.
I've been pondering this question for a long time. I'm also drunk, so I want to write this down for future review. I'm sure many are well and eager to elucidate the wrongness of my points, so have at thee.
So, what's OT having for breakfast tomorrow? I'm thinkin' Wheaties.