D&D is a game, plain and simple. You could run through the streets slaughtering every person you come across, and all you’d have done is drop a piece of paper’s hit points down to zero. You haven’t killed a person, you haven’t committed a sin, you’ve just played a game.
That being said, I’ve never been able to play an evil character in an RPG… except once in Fable 2…because I don’t even like entertaining the notion of killing innocent people. I don’t think it’s sinful to kill a game character, but I just don’t like the idea. If the idea of killing a sentient RPG creature bothers you, then don’t kill them.
That being said, almost all of the sentient monsters in D&D are thoroughly evil. Goblins kill indiscriminately, succubi drain the souls out of people, vampires kill people, kobolds swarm cities… etc. If if were a human doing these things, I think we’d universally agree that it would not be immoral to put an end to their actions, even if that end was brought about by their death. Applying that same principle to the D&D creature, since they are almost invariably awful, it would be licit to kill them to prevent them from harming other players or NPCs.
Again though… I’d like to stress that this is a game, and so long as it doesn’t lead you to think that you can kill people and animals in real life, I wouldn’t worry about it.