It depends a bit on how u look on these things. I personally killed some dream characters, did i feel guilty? Not really, but that's because i know it was a dream. For me it feels the same as killing somebody in a videogame, i know it's not reality. And like some people sayed u can just wish them back. I don't really see the big issue.

But then again, if u kill somebody that u hate for example in real life. And u keep doing this, cause it brings u so mutch satisfaction. It may bring those actions/thoughts into real life. And that's unhealthy ofcourse. And i would advice to seek some help right away.

It's all about do u know the difference between dreaming/reality? If yes, then don't worry about it. Don't get me wrong, not saying u should kill in every lucid dream. To mutch of something may bring u harm in some ways. But if it's once in a while, i wouldn't worry about it. I think it may even help u release some anger, afther a stressfull day. But that depends completely person on person though.