I read Osprey's post on Karma this morning, and it spawned a whole set of thoughts on that concept.
In particular, I also believe karma is based upon action, but some actions are very subtle ... thoughts can be actions, in my view. A stray thought or image about killing or having sex or how horrid someone is ... I don't believe those things have much impact outside ourselves. It is focusing on the thought, expanding it, imagining what it would be like, making it real and vivid in our minds. That generates karma, I think.
I guess to see a strong thought or image as having karmic repercussions, one must believe in telepathy, and I have experienced that function in animals, plants and humans too often to discount it.
Like everyone I have destructive thoughts. The trick is to move away from them and release them. The memories of what I felt was harm being done to me could generate fantasies of revenge, but I try to keep away from that and focus upon what will be positive for my future.
Obsession is the extreme result of persistent thought and imagery on a subject, but there are many degrees. Controlling our thoughts is one of the admonitions of The Wise. I try and fail, and try again. There is no other way to work with it but to keep trying.