It's a countable noun only.

elanvital, Noncountable: I wrote a paper about massacre as a tactic in war. Countable: Bodies were lying everywhere after the poisoning. It was a massacre and the third massacre of innocent animals in Texas this year. Many nouns can be uncountable when they define activities but are countable when they refer to a category or unit. Noncountable activities. I ate breakfast. I went to school It was war. Countable categories: I want a breakfast like no breakfast I have ever tasted before. I went to a school that had no teachers, only students! No one wants a war with the Martians.