His act of betrayal was a breach of trust toward the company.
違反 is a general term for violation or infringement, while 背反行為 specifically emphasizes an act of disloyalty or breach of trust.
背信行為 focuses on betrayal of trust, often in personal or professional relationships, whereas 背反行為 can also cover broader violations of rules or expectations.
Compound of 背反 (betrayal, violation) and 行為 (act, deed). The exact historical derivation of 背反 is uncertain, but it is conventionally associated with the meaning of going against or turning one's back on something.