Your comment is somewhat hard-hearted. Isn't anger one of the early stages of grief? No matter how twisted the father's politics, the man just lost his son in an unimaginably horrible way. I for one will reserve my judgement until we know more about this case.
Had his anger been directed at both the people who beheaded his son as well as the President, I would have chalked it up to mere grief. However, he went out of his way not to demonize those who actually took off his son's head and used the death to play politics.
Berg's father was involved in socialist, anti-war organizations way before his son went to Iraq.