Why not kill your neighbor? After all, there's no telling when he'll get it in his head to come kill you. Might as well go ahead and murder anyone you bump into, they might be a murderer themselves!
When your neighbor is a different species, this is pretty much exactly what humans do: kill it. The more unfamiliar, the faster it gets killed.
Is it bad for the ecosystems, biodiversity, your soul? Yes on all counts. But it's pretty consistent human behavior.