General:
* Slaughter: This is the most common and direct synonym for "massacre." It implies a large-scale killing of people or animals.
* Carnage: This word emphasizes the brutality and bloodshed of the event.
* Mass killing: This is a more neutral term, focusing on the large number of victims.
* Bloodbath: This word is highly evocative and emphasizes the violence and bloodshed.
* Atrocity: This emphasizes the cruelty and inhumanity of the act.
More specific:
* Genocide: This refers to the deliberate killing of a large group of people, specifically targeting a particular ethnic, religious, or national group.
* Pogrom: This specifically refers to a violent attack on a Jewish community.
* Massacre of [place/group]: This is a direct and specific way to refer to a massacre, naming the location or the group targeted.
Ultimately, the best noun to use will depend on the context and the specific message you want to convey.