I've been there, I also had a therapist laugh at me for actually the very same thing a few years ago. I left his service and never went back. It actually took me being hospitalised a few years later (because things had gotten so bad) for me to get a new PDoc and by God she's a million times better.

She sent me for all sorts of referrals (which wasn't a fun process but it was worth it) and I eventually got diagnosed with Aspergers, OCD, and a Dissociative Disorder. From what I've come to understand, having Aspergers actually puts us at a much higher risk of mental health problems such as these and any therapist worth their salary should take any reports like this very seriously indeed. The graphic violent thoughts sounds very like what I get from my OCD, I have a few 'branches' of OCD but the primary problem I have with it is intrusive thoughts. I also have body symmetry OCD but that's a whole tapping nightmare of difference.

I'd highly recommend searching for a new PDoc, preferably one who has experience with Aspergers.