Why was the father (I forgot his name) so enraged when he saw the photo of the son's girlfriend on the wall? Did he, as his wife claimed, have the hots for his son's girlfriend?

Up to that point in the film Richard had been demonised in the thoughts of Frank's parents. They despised Richard and were frightened by him. They saw Richard as a monster incapable of having a normal relationship with people. He was a bully and a thug at the beginning of the film and degenerated into a murderer. They had nothing but loathing for him. When Dr Fowler saw the picture of the once normal loving relationship captured in that photograph he saw Richard as a normal human being who was once loved by Natalie. This image disturbed the world picture held by Dr Fowler. While he held Richard in contempt it was easy for him to comtemplate murdering him because Richard was not a human being. Seeing Richard as a happy man brought home to Fowler that he was once like him; a happy and contented father. Richard was no longer the monster but a flawed man fighting to keep his happiness...just like Fowler. If Richard was a monster so too is Fowler.