The baby ocelot that spawns with baby witches is unkillable for some reason. No matter what you attack it with, it just doesn't lose health. This effectively makes it an eternal nightmare that follows you wherever you go at inhuman speeds, permanently pecking away at your health until you die. Because it never can, and never will.

Currently, the only way to fix this that I know of is using a mob blacklister mod to prevent it from spawning.