If there were say merpeople from a similar time table it would take a long time for conflicts to start because they have a totally different environmental niche.
Anyway early bands of hominids were nomadic by necessity due to localized depletion of food supplies. Given that scarcity it is no wonder that they would come into conflict and managed to drive megafauna to extinction. And their numbers weren't exactly high - humanity bears genetic scars of near extinction.
Said scarcity is such a thing of the past that we have food distribution problems instead of production ones.
Granted there is also a long history of zero sum thinking holding humanity back.
Edit: and going back to you merpeople example, the onset of humans represents a mass extinction level event for the closest thing to merpeople: ceteceans. http://www.iucn-csg.org/index.php/status-of-the-worlds-cetac...
Bacteria could wipe us out tomorrow given the right circumstances. They can be assholes too.
Not really; they don't have any cognition. They're just little machines. They're no more of an asshole than a toaster or a clothespin.