I wouldn't wish death on somebody I hated, but I also wouldn't complain if they were to die. Wishing death on someone who then dies would no doubt lead to baseless guilt, which would only make me feel worse as opposed to better because it'd make me feel as if my wish was the deciding blow.
That being said, there's no actual harm in wishing for the death of a bully. It makes you feel better (if it didn't, you wouldn't wish it) and the bully wouldn't care because to them it's just words from a lesser being. Actually acting on it and killing the bully would be a big problem, though.