General synonyms:
* Deceive
* Double-cross
* Treachery
* Duplicity
* Dishonour
* Violate
* Break faith
* Fail (in a moral sense)
* Go back on one's word
* Stab in the back (idiomatic)
Synonyms emphasizing breaking trust:
* Forsake
* Abandon
* Desert
* Deny
* Violate a promise
* Break a vow
Synonyms emphasizing deception:
* Mislead
* Lie
* Deceive
* Trick
* Swindle
* Manipulate
Synonyms emphasizing betrayal in romantic relationships:
* Cheat
* Two-time
* Have an affair
Synonyms emphasizing betrayal of principles:
* Renege
* Abandon
* Compromise (in a moral sense)
* Go against one's beliefs
More formal synonyms:
* Perjure (to lie under oath)
* Prevaricate (to evade the truth)
* Subvert (to undermine)
* Traduce (to slander)
The best word to use will depend on the specific context of the situation.