"Massacre" is an original score composed by Christophe Beck for the Buffy the Vampire Slayer episode "Becoming, Part One." It plays between the minutes 36:29 and 40:26 of the episode, while Angel distracts Buffy from the attack Drusilla leads against the Scoobies.
The music is also used in the Angel episode "War Zone." It plays between the minutes 16:18 and 17:30, while Angel is running away from Gunn's crew.