More people are killed by what?

People themselves probably kill each other more than any other animal harms people.

However, to my knowledge, mosquitoes kill more people than any other animal on Earth by spreading diseases such as malaria and yellow fever.

Note that mosquitoes are pollinators, and thus have an important role in ecosystems. So they should NOT be completely exterminated. It is just important for people to protect themselves when they enter mosquito ridden areas, and for people to prevent mosquitoes from breeding where lots of people live.