Harm thoughts are the hardest for me to get past as well. Like you, I struggle with the "what if I start WANTING to do these horrible things?!?" type of questions. It absolutely horrifies me, but its even worse when it doesn't seem to bother me as bad. That's when I start wondering about my own good nature and intentions and almost become convinced that I'm some terrible, ugly being.
In reality, we don't want to be anything close to what our fears keep whispering to us. You've defeated it before, you can do it again! It may have put on a different mask, but it's still the same lie.
BTW, I just started reading "The Imp of the Mind" by Lee Baer,PhD. You may have already read it, but if you haven't, I recommend it. It's been a great resource for me so far when it comes to intrusive thoughts and obsessions.