The term "killer of bulls" can be replaced with several different synonyms, depending on the context of the sentence. For example, "bullfighter" is a common term used to describe a person who participates in the traditional sport of bullfighting. Another synonym that could be used is "torero," which is a Spanish term for bullfighter. Additionally, "matador" is a well-known term in the bullfighting world and is often used to refer to the person who kills the bull in the final stage of the fight. Other possible synonyms include "tauromachian," "picador," and "banderillero," all of which are related to the practice of bullfighting in some way.