‘Mass murderer’ says it better 

Actually, that word became popular only in fairly recent times, and “mass murderers” says it better. The word “terrorists” was invented to designate unpleasant people who try to kill other people whom they dislike or disagree with – usually in large numbers, usually quite brutally, and usually because of political, religious or ethnic differences. Vocabulary evolves to keep up with the ongoing march of human depravity.
We do have a term to designate people like Hitler and Stalin:  state terrorists, or those who use the apparatus of the state to commit atrocities on a large scale. But “mass murderers” says it better.
Ye Olde Pedant