If they were real, than Twilight couldn't cast any spell because magic doesn't exist. You have to recognize that the MLP universe doesn't work the same way as ours and we can't compare it to our universe, otherwise it kind of ruins the fun of a fictional universe. The only way to see how it does work is to look to what the show and canon material tell us.
I feel like you are just stuck on the fact that I mentioned it's a TV show. You are one of those fans who hates when people use the "it's a kids show" excuse to answer a question, am I right? I'm not trying doing that. Sorry if it came off that way. I'm trying to establish if there are things within the MLP universe that would be impossible with magic. The thing is that because there really aren't many canon explanations of that, I have to think about what the creators would possibly make canon, and like it or not, the fact that it's a TV show has an effect on that.
True, just because they won't show something on the show doesn't mean it can't be canon. Like I said before, when we see unicorns shoot those magic beams, those beams would probably have the power to seriously injure someone or even kill them. They won't show that or make it canon, but that's probably what would happen. But because of the nature of the brand, they have to make sure everything they make canon is appropriate. Therefore, what is canon vs what is non-canon is affected by the fact that it is meant to be a kids brand. Because of all that, I think that it is reasonable to think that if the creators (the people who decide what is canon) were asked if there is a spell for killing in MLP, they would possibly say you can't kill with magic, or at least a spell like that doesn't exist.