Hi there, electricswitch.
I don't think your fantasies as you're describing them here are necessarily a cause for concern. It isn't unusual for people to enjoy sexual fantasies about things they wouldn't like to experience in real life (or that they'd only like to experience in specific circumstances).
Having violent sexual fantasies doesn't necessarily mean that you're going to be more likely to perpetuate sexual violence or harm someone else, especially since it sounds like you have a pretty clear understanding of what isn't going to be safe or okay to do with someone in a real sexual experience. And I think it's worth pointing out that you say these fantasies are better when you're picturing a consenting partner, too.
At the same time, I can understand being a little worried about these fantasies, especially if they're new. If it feels better for you to focus on less "extreme" fantasy subjects for now, I think it's fine to try and do that. But it's often very difficult to change one's fantasies altogether, and it isn't something I think you have to try and do in this case.
This advice column has some additional thoughts about BDSM and more "extreme" fantasies that you may find helpful, as well.