When trying to find synonyms for the phrase "act the traitor," there are a few options available. One common phrase often used in place of "act the traitor" is "stab someone in the back." This implies that the person is going against someone they had previously been trusted by turning on them unexpectedly. Another option is "betray," which can be used in the same way as "act the traitor." A third option is "double-cross," which has a slightly different connotation but still implies a betrayal of someone's trust. Overall, there are several phrases that can be used in place of "act the traitor," each with slightly different connotations.