Activation base is in stamina and values vary with fatigue%.
Melee only: This talent will not work when a ranged weapon is equipped.
Despite the description, this talent does not necessarily kill lower rank enemies automatically. It just deals Area of Effect damage approximately that of a double critical. Therefore, warriors with two-handed weapons and high strength values tend to get the most out of it, as will dual-wielding warriors making use of many intensifying runes. The precise calculation formula is unknown at the moment, but examples for average damage values can be given: A level 27 warrior with ~60 Strength wielding a one-handed sword and shield deals about 160 points of damage on average, while a level 34 warrior with ~160 strength wielding a two-handed weapon deals almost 500 points of damage on average. See also the discussion on the BioWare forum
While the description suggests some kind of whirlwind-like weapon attack, the animation for Massacre is in fact identical to War Cry, i.e. the character is standing still while unleashing a battle shout.