To reward or recompense—repay someone's actions or efforts (often used in the passive).
彼の努力に報いるために昇進させた。
He was promoted to reward his efforts.
親切に報いるのは人として当然だ。
It's only right to repay kindness.
長年の貢献が報いられて、表彰された。
Years of contribution were rewarded and he was honored.
To retaliate or take revenge—responding to harm or betrayal with retribution.
彼は裏切りに報いるために復讐を企てた。
He plotted revenge to retaliate for the betrayal.
被害者は自分で報いることを選ばなかった。
The victim chose not to take matters into his own hands to retaliate.
物語では正義が最終的に報いることが多い。
In stories, justice often ultimately rewards or repays (someone).