mirror neurons. It's also why you can see somebody throw a rock and know how to throw a rock, and why seeing a good limb in a mirror box (providing the illusion of your amputated limb being back) relieves phantom pain. You see the limb, wriggle it around, the missing feedback is provided, all is well in the world. Brains are weird.
We're hard-wired for empathy, but of course, we can turn it off. Actually, if you think about it, humor might be critical to this process (i.e., turning off empathy.) effed up thought there, but what are our two reactions to these clips? "HA HA" or "Owwwwwww" (I think "Ha Ha" happens more when it somehow "serves him right." We're safe from the outcome because the victim has behaved in a way that we just know we wouldn't.)