I thought it would've been better if Diaz hadn't actually set the guy on fire -- if he'd decided that just seeing his childhood bully so broken and powerless, and leaving him to live with that knowledge of his weakness, was the better revenge. If he'd realized that he hadn't needed to kill the guy because he'd already conquered his demons. He would've been a more impressive figure if he'd done that. The fact that he went through with it anyway showed that he's not as smart as he thinks he is -- that he ultimately is just a thug, and that his fear still controls him.