Do you know what the world's most deadly animal (for humans) is? The grizzly bear? Or wolves? Or maybe the humble hippopotamus? There is a streaming video below the fold that answers this question. [1:58]
The females of most mosquito species suck blood (hematophagy) from warm-blooded animals. This has made mosquitoes one of the most deadly vectors known to man, killing millions of people over thousands of years and continuing to kill millions per year through the spread of diseases.
From the title, I was tempted to say "Humans."
Even with the "(for humans)" restriction, its probably still a close call between humans killing humans and mosquitoes killing humans.
I think this is unfair to the mosquito. It may spread disease, but it doesn't kill anyone itself. It is as much a victim to the parasite as we are. The malaria parasite is the real killer, but it doesn't qualify as an animal being on the wrong branch on the tree of life.
So what is the animal that kills the most people itself rather than just as a carrier of disease? I'd guess one of the parasitic worms.
I think we need a more accurate definition of "killing." Who's the real killer, the guy holding the gun or the bullet? The mosquito or the disease? The grizzly bear or the bear's teeth? Etc.
I don't disagree that the mosquito is the deadliest animal, as it is the mosquito's behavior that directly causes the spread of malaria. What I'm really curious about is: which animal (humans included) has intentionally killed the most humans?
Once the nukes start going off, this discussion will need revision, if there is an Internet afterwards. Did anyone see the film version of Stephen King's THE DEAD ZONE? Remember when the lead character has a premonition about a political candidate who will reach the presidency and launch a nuclear attack just as the diplomats have resolved an international crisis, shouting, "It's too late! The missiles are flying! The missiles are flying! It's my destiny!" I can see some of the Religious Right wackjob wingnut Republican politicians doing something like that while clutching their King James Bibles (the ones in which Jesus' purported words are printed in red).