godan verb (-ru), transitive verb
betray; double-cross; be unfaithful to
Used for betraying a person, group, trust, or expectation. Covers treachery, infidelity, and breaking promises.
彼は親友を裏切って秘密をばらした。
He betrayed his best friend and revealed the secret.
味方
I would never do anything like betray my allies.
godan verb (-ru), transitive verb
disappoint; let down; fail to meet expectations
Used when someone or something fails to live up to hopes or expectations. Often used with 期待 (expectations).
彼の行動はみんなの期待を裏切った。
His actions disappointed everyone's expectations.
結果が予想を裏切って、とても残念だ。
The result fell short of expectations, which is very disappointing.