I can't find a link right now, but I remember an interview with David Simon where he mentioned that Omar was originally supposed to appear only in a short arc (and then be killed). The actor (Michael K. Richards) was so good that they had to keep him. (Still can't find a link to the David Simon interview, but here's a link to an interview with the actor. He says 7 episodes was the original number.[1])

My point being: that character had to die. As awful as I found it to watch his last episodes, there was really no other end for him.

[1] http://www.hobotrashcan.com/2005/08/23/one-on-one-with-micha...