This usually comes up during Halloween time when they want to make their pet sound scary. In this case it is their cat. But sometimes people just want to really name thir cat a spooky sounding name.
In this case, you can bascially look up any negative word in the thesaurus and start from there. That will lead to other negative words for you to choose from.
I'll list a few here, but if you don't like any of them, then just find other words that are similar and that should give you more ideas.
Bones, Reaper, Fetid, Despair, Pandora, Claw, Grim, Fang, Evil, Raven, Black, Shadow, Salem, Hades. . .
Also, another common way of finding scary names for your cat is to use vampire, monster, or demonic names. These would include names from Stephen King books for example, or demon names from religions.
Lucifer, Vlad, Lillith, Satan, Damien . . .
Like I said, if you don't like any of theses names, just put them into an online thesaurus and it will bring up lots of different suggestions for you. that way you can have a more unique name because most of the ones here are used by many people.