It's bad to scare someone you know is vulnerable.
Dean's older brother Graham has returned home from the military after being diagnosed with PTSD.
Dean wants to play a prank on Graham for April Fool's Day.
Dean switches around the contents of his and Graham's cereal boxes.
Dean buys a fake gun and pretends to be a robber breaking into Graham's room.
Graham is surprised to pour the wrong cereal and shares a laugh with Dean when he realizes what happened.
Graham's PTSD is triggered, and he reflexively hurts Dean because he thinks his life is in danger.