Why some mites are mightier than others: The evolution of lethal fighting in a spider mite

While fighting over females for the right to breed is common among male animals in the wild, these fights rarely result in death. You can't pass on your genes when you're dead, or badly injured. So why do the males in some colonies of the tiny spider mite display aggressive, and sometimes deadly, behavior against rival males, while the males in other colonies do not, and how did this behavior evolve?