Saying that humans kill a large number of humans doesn't mean that we can wipe out "all life on earth". We do a pretty good job exterminating large mammals which is what we ourselves are, after all ...
I would say that humans have killed the most humans. Mosquitoes are a clever answer but insufficient, because mosquitoes don't kill anyone - they just pass on GERMS that can kill you. (and there is such a variety of germs that none of them should deserve the prize). For that matter, HUMANS can pass on lethal germs. Most of the people killed during the Black Death, for instance, weren't infected by rat fleas, but by inhaling plague bacteria coughed up by other humans.
If we don't count humans or disease-carrying creatures, then probably domestic dogs have killed the most.