A double-crosser is the person who double-crosses you: someone who cheats a partner, especially after you had already agreed to do something dishonest together. You reach for this word when you want to say they cannot be trusted — not even by the other people who were already bending the rules. It is informal, strong, and a little film-like. It names the betrayer, not the act (the act is a double-cross). Though actually, people sometimes loosen it and call any back-stabber a double-crosser, even when no “crime together” was involved; the core picture is still “sold out their own side.”