Well... there's my old friend I'm always linking members here to... this mental-health-oriented description of the practice of compassionate abiding:

https://mindsetdoc.wordpress.com/201...e-abiding-101/

Beyond that, I think this is probably something you simply need to work through with the help of a skilled therapist. Presumably there are experiences you've had in the past that have stayed with you, perhaps stored in non-conscious areas of your brain, that are causing you to have these homicidal thoughts. I know I struggle with a lot anger as well. So I know it's not pleasant. Physical exercise (walking & yoga) & meditation are helpful for me as well.