Ending (Spoilers)

I believe it was his ultimate payback to Gilbert. He saw Gilbert as a manipulative and conniving person after finding out about Val was only with him b/c she didn't want him to die while also on the sideline having as secret relationship with Gilbert himself. Matt blames Gilbert for "forcing" Val to betray him like that, and perceives that Val would've never gone along in being his girlfriend if Gilbert hadn't made her.