Flippy has Post-Traumatic Stress Disorder, which causes him to "flip" out and think he's back in the war whenever he's reminded of the war. When reminded, his pupils dilate, his irises becomes yellow-green, his voice goes from squeaky to a deeper, darker voice, and he becomes extremely violent, losing any shred of empathy he had and committing murder. It is also possible that he has Dissociative Identity Disorder also known as Multiple Personality Disorder.