Single word for "trust breaker"?
The words "turncoat" and "deceiver" can be used to mean "trust breaker," but both have a connotation of betrayal.
Traitor and Betrayer are the two words I can think of other than the ones mentioned in the above answer.
The strongest single term I can think of is "perfidious", relating of or to "perfidy" - the deliberate breach of trust, or betrayal.
Other words would include "oathbreaker" (arguably a compound word, "oath-breaker"), or traitor.