Off the top of my head: Ruckus, Chaos, Havoc, Thor, Loki, Bruiser, Diesel, Thunder, Razor. Also take a look at mythological creatures and gods. Maybe look for historically well known criminals, ie. Clyde, Manson, Dahmer. A bit morbid, but it sounds like that's the effect you want. I just hope you want a vicious name, not a vicious dog!
Some good names to consider when looking for a vicious pet name include those used for heavy metal bands (Vitis, Detritus, Venom, Helheim, Thyrfing come immediately to mind as good pet names). Or use a Thesaurus to find alternate versions or play with the spelling of often used pet names. Or, go with "Muffin" for a slathering pit bull name.
