The greatest killer ever: prey incapable of defending against predators. No doubt about it.
"Gordon Gecko" was wrong (from the film, Wall Street). Greed might well have been what has marked the upward surge of Man, but I think it's fair to say that it has been the ability to protect oneself and one's family that started it all and has kept it going. Couldn't get off the ground without that. And potential prey can't survive predators very well without that. Stripped of that, Man is juts another prey animal ... of other animals, felons, or vindictive enslaving government hirelings.