It might be a good idea to talk to a therapist about this, who can answer your questions in a tailored way and explain why ERP works in detail?
But basically, it's not 'fixating' on a violent or murderous thought in a harmful way. It's a proven technique to unstick a temporarily misbehaving loop of the brain. When you have OCD, a slightly damaged/wonky/inflamed/stressed bit of the brain keeps sending you an irrelevant thought, under the false impression it means something important. By embracing the thought in a non-judgemental way (as in, not getting angry or frightened at your own brain for making the mistake), you show it there's nothing to be frightened of.
There is zero evidence that having a criminal or violent thought will actually turn you into a violent criminal. Virtually everyone has the odd randomly violent or angry thought. It's normal. Things like "I'm so angry with my boss today I could just punch him!" or "imagine what would happen if I stabbed my dog with this chopping knife". It's just in people without OCD, the thought pops in and out of their mind so fast they barely notice or remember it, whereas in people with OCD the thought gets 'stuck' on repeat.
Yes, it's true that people who commit acts of terrible violence are often very interested in violent online videos, aggressive scenes from movies, fantasising about hurting others etc...but since these people don't have OCD, their minds are working in a completely different way. So it's a completely different situation. They're deliberately seeking out violent and criminal content and actually enjoying it - whereas people with OCD are terrified and aren't enjoying these thoughts at all.
There are also loads of examples of people thinking about murderous or violent things and it NOT turning them into anything different. The context really matters. For example, I'm a writer and some of my book plots have involved researching weapons or criminal violence online, thinking deeply about these, and writing a character comitting them. This hasn't magically made me a violent person 🙂 I generally have an anti-war political stance, I've never been in a fight, I have zero criminal record, I don't own any weapons, and I believe firmly in non-violent peaceful activism. Police detectives, social workers, true crime podcasters, writers and filmmakers, doctors etc all have reasons to think deeply about criminal violence in different ways, and it doesn't turn them violent either.