Antonyms for the phrase "backed you" could be "opposed you," "disagreed with you," "refuted you," or "challenged you." The antonym for "backed" is often associated with being against something or someone, rather than supporting or endorsing them. It is important to note that disagreement or opposition does not necessarily mean hostility or animosity; it can simply imply a difference of opinion. In certain contexts, constructive criticism or pushback can actually be beneficial for personal growth and development. Hence, having people in your life who are unafraid to challenge your ideas can be a valuable asset.