To betray or turn traitor to someone; to double-cross or be unfaithful to a person or cause.
彼は親友の秘密を話して友達を裏切った。
He betrayed his friend by telling his secret.
敵に情報を渡して国を裏切ったと非難された。
He was accused of betraying the country by passing information to the enemy.
仲間を裏切るような行為は許されない。
Actions that betray your comrades are unforgivable.
To disappoint or let someone down; to fail to meet expectations.
この映画は評判ほど良くなく、期待を裏切った。
This movie didn't live up to its reputation and disappointed me.
約束を守らなかったことで彼は彼女の信頼を裏切った。
By breaking his promise he let her down and betrayed her trust.
天気が予報を裏切り、遠足は中止になった。
The weather went against the forecast and the outing was cancelled.