By Vassia Barba The man who showed the world the severed head of his father in a YouTube video had “gone off the rails” in college, people who knew him claim. Justin Mohn, 32, has been arrested on charges of first-degree murder and other counts, following the gruesome discovery of his father, Michael Mohn, decapitated in their family home in Middletown Township’s Levittown section in Pennsylvania. The shocking incident unfolded on Tuesday evening, with court records revealing the grim details of the crime. Mohn’s arrest comes after a disturbing YouTube video, posted and subsequently removed on…