Justice or Revenge
Cooper was a schmuck. No he was the schmuck of schmucks. I couldn't believe how he made light of Sawyer's pain and tore up his letter!! On the island, where no governing body exists (unless you count Smokey :-) ), was Cooper's death justice or was it revenge? An execution or a murder? Was it the ethically, morally, the right thing to do? There's no clear answer here so I hope many people have opinions each way, maybe both at the same time. That's how I feel.
Furthermore, why did Sawyer react so violently to something he'd been willing to do before? Why was Sawyer so reluctant to kill? Why did he fight it until Cooper egged him on? Why was he so angry with Locke even at the end? How do you think killing Cooper affected Sawyer? What would you have done in his shoes?