To repay kindness with harm; to return evil for good — an idiom meaning to betray or harm someone who helped you instead of repaying their kindness.
助けてあげたのに、君は恩を仇で返したとしか言いようがない。
I helped you, and all I can say is you repaid me with evil.
恩をあだで返すような行為は決して許されない。
Actions that repay kindness with spite are never acceptable.
彼は恩を仇で返すという悪評が立っている。
He has a bad reputation for repaying kindness with harm.