A Pennsylvania man accused of murdering his father seeminglyshowed off his decapitated head in a grisly YouTube video — also rife with calls for revolution, political violence and condemnations of the Biden administration — hours before his arrest. Justin Mohn is facing charges of first-degree murder, abuse of a corpse and possessing an instrument of crime with intent in connection with the slaying of his father, Michael Mohn, Middletown Police Capt. Pete Feeney told NBC News. He was taken into custody Tuesday night after authorities were called around 7 p.m. to a home in Bucks County, where t…