human beings

This is an easy one. Human of cource. Most of the species that usually get associated as the world's most dangerous predator for example Tiger, Great White Shark and Polar Bear are endangered species nowadays thanks to humans hunting them down. They might have gona extinct already without conservation measures. And humans are damn good in hunting and butchering one of their own too. And animals kill for necessity like: food, protection, defence of their offspring. Humans can kill for the sheer fun of killing. Killing seems to be self-virtue. Or humans kill and wage wars in the name of absurd abstract ideals...politics and religion just to name few of them or just by pure intolerance...which usually breeds from the one of two previously mentioned. They also might be the only species who have dangered the balance of the whole planet's ecosystem. Of cource some species have caused lot of local enviromental damage but humans are most likely the only planet catastrophe.

Wow, I think that from all these questions, you must have a lot of homework.