He is a former soldier who became a member of the Blue Brigade from the frontline fighters for East liberation. In order to gain more power for himself as a soldier, he obtained an auto-mail left hand. However, he was cast aside for doing such a thing.
He is an idealist fighting against a military too powerful to defeat. As a result, he turns to terrorist tactics to try and gain support for his cause.
Bard seems to have a score to settle with Mustang even though he does not even stand a chance.