Fetching details from the dictionary...

rival; opponent; adversary; competitor; enemy (esp. one with which there is longstanding enmity); foe
revenge
父の仇を討つために旅に出た。
He set out on a journey to avenge his father.
あの二人はライバルで仇同士だ。
Those two are rivals and adversaries.
恩を仇で返すようなことはしたくない。
I don't want to repay kindness with ingratitude.