A Philadelphia judge ruled Thursday that Justin Mohn, the man accused of beheading his father and posting a video of it online, is competent to stand trial. The decision came after a five-hour hearing where Mohn, smiling and nodding throughout, dismissed his public defender and requested new counsel. Dr. Kelly Chamberlain, a forensic psychologist testifying for the prosecution, described Mohn as intelligent, calm, and socially appropriate during their two meetings. She disputed the findings of defense expert witness Dr. John Markey, who diagnosed Mohn with delusional disorder, citing letters w…