I think it depends on the situation. If the killer did it in one event, he'd be a mass murderer. If he killed 100 people over the course of years, one at a time, he'd be a serial killer.

mass murderer

mass murderer or "serial killer" or "war criminal" if committed in a war/conflict. or "a dictator" if the head of a country that is acting badly etc.

"Mass murderer" is the first phrase I thought of. There is no phrase that resembles "hundred-bodied killer." If you want to stay close to the idea of "a hundred," you would simply say "he has killed hundreds of people" or "he is known to have killed hundreds of people." In the specific case of a criminal who works in an organized crime syndicate, whose paid job is killing people when his boss assigns him the job, there is a slang phrase "hit man" (because killing someone for hire is "making a hit.") "Hit man" doesn't carry the idea of "hundreds," it carries the idea of "a job, for pay."