This trophy is, obviously, related to killing enemies in brutal fashion, but it is unclear whether you need to just kill enemies in the standard manner, or whether you need to kill enemies with the brutal QTE kills when they have the "circle" prompt over their head.
Fortunately, there is a sure-fire way to get this trophy. If you have not unlocked the trophy by the time you reach the final boss battle, here's your chance to get it. If you don't want to see some minor spoilers for the final battle, then don't open the hidden spoiler below:
*** Spoiler - click to reveal ***When Zeus goes down for the final time, he will get the standard "circle" prompt over his head. Approach him and press "circle" and you will be prompted to repeatedly mash the "circle" button. As you pound away on Zeus' face, the screen will become more and more covered with blood. This QTE will actually not end until you get tired of thrashing Zeus and stop mashing the button, so just keep vengeancing the shit out of him until your trophy unlocks.