Odd as it may sound, I think it's possible to have both homicidal desires and harm OCD.
OCD isn't terribly rational, and if you perceive something as bad or a threat for basically any reason, even just some of the time, it can potentially fuel OCD.
I say this because, uh... personal experience. With both. Overlapping.
edit: To clarify, this was because the reason behind my harm OCD wasn't "harming people is bad," but more specific to the form of harm and what it implied about me as a person. Basically, there were reasons I should not take certain harmful actions or think certain thoughts about harm. But I still wanted to harm people, I just had to do it in very specific ways or it'd violate the rules laid out for proper behaviour as dictated by my obsession.