Humans have inhabited the planet only a few million years ago. Mosquitoes have killed way more species than humans. Humans are just the most vulnerable animal not the deadliest.
What makes mosquitoes so dangerous? They carry devastating diseases. The worst is malaria, which kills more than 600,000 people every year; another 200 million cases incapacitate people for days at a time. It threatens half of the world’s population and causes billions of dollars in lost productivity annually.
They don’t kill humans directly but rather spread diseases like malaria, dengue, yellow fever, etc. Every year, more people are killed by mosquito-borne diseases than by any human action — war, murder, etc.
The biggest killer in the developing world is presently HIV/AIDS, but now we enter the question of whether or not a virus counts as a living thing. Then you have the problem that HIV/AIDS make other diseases, including malaria and tuberculosis, more deadly.
Humans have the capability of creating, managing, and destroying. It’s just the way you look at everything. Beauty or ugliness lies in the eyes of the beholder.
It’s all in the mind!
With all of this information, you can feel free to choose for yourself which is the deadliest animal or microbe or thing, and argue your case, but note that not everyone will agree with you.