would-bero

A would-be hero, someone who tries to promote themselves by claiming afterwards that they could and would have done something heroic if some (usually minor or irrelevant) factor had been different.

The would-bero claimed that he would have chased down and used his kick-boxing skills to stop the thief who mugged the old lady, if only he'd been wearing his old sneakers rather than the new ones he'd just bought at the time.