No matter how bad a place is, there will always be some lifeforms that can thrive. Give it enough time, and you almost won't be able to tell how bad the situation was originally. Sometimes, they'll just keep multiplying like crazy. However, due to their environment, they also happen to kill like crazy.
No matter how many die, unless all life is wiped out, it is impossible for Earth to be peaceful. That's just how it was designed.