I suspect that Omar signed his own death warrant this week. Correct me if I'm wrong, but Omar has never killed for sport before, never murdered an innocent. Savino isn't a choirboy, but he never wronged Omar directly. By doing Savino, cold-blooded, on the street, Omar betrays his own code. He's no longer a sanguinary angel, just an outlaw gangster. He may still have his revenge on Marlo, but he may have lost his halo of protection.
I've seen a similar trope on other Wire blogs this week. But I think this notion loses sight of two things:
1) Omar has already said that he's going after all of Marlo's people, especially his muscle. Earlier in the same episode, we've seen him kill one of Marlo's guards just to flush the stash. At this point, it's not about who Omar kills; he's only leaving people alive to tell his story.
2) Savino, as Omar remembers, was muscle for Avon Barksdale. In the first season, Avon had Omar's boyfriend Brandon tortured and killed. Omar knows that Wee-Bay, Stinkum, and Bird were involved in Brandon's death; he's killed Stinkum, put Bird in prison, and killed Stringer Bell, while Wee-Bay and Avon are likewise in prison. Savino is the last person on the street who could have had anything to do with Brandon's death.
P.S.: Mouse over for a spoiler.