The word sacrifice derives from the latin sacrificium, meaning a sacred act. It originally referred to making an offering to the gods of a slaughtered animal. That’s how Brutus is using it here. He’s comparing the killing of Caesar to a religious offering to the gods, which would bring benefits to the Roman people. That’s also what Brutus means a couple of lines later when he says that they will carve Caesar “as a dish fit for the gods.”