Perhaps he learned it from some other TV show you had on? Kids are sponges and pick stuff up when you think they're not paying attention. If you're letting him watch violence and shows about people being shot in the face, perhaps he's also watching shows where people are rude and cruel. After all, shooting someone is pretty much an extreme of rudeness and cruelty.
Keep in mind that even innocuous-seeming sitcoms are full of rude, mean people saying all kinds of insults to each other. Even some news networks are filled with rude pundits shouting at each other, telling each other to "shut up", and being generally disrespectful. These examples almost never have any kind of reprisals for the bad behaviour, and in some cases it's encouraged or even glorified.
If you have any kind of TV on that's not suitable for kids, you can expect your child to pick up on inappropriate habits.
Also, you need to find out why he said the face was yucky. The fact is, whether it's politically correct to say it or not, humans have an inborn sense of beauty and studies show that it's largely based on symmetry. An asymmetrical face - especially due to injury - is less appealing. As civilized people we learn to not be rude about such things, but admit it...if you were walking down the street and saw someone with a severely injured face, you'd think, "Oh, poor person, that's awful," but you wouldn't think, "How wonderful that that person is empowered and above lookism to be able to go out in public like that." The latter statement is what political correctness would say, but it's nobody's gut reaction.
So the thing to do is to teach your son how to be polite and civil in such cases, not necessarily to deny his observation.