In this episode the Ori help these people construct a weapon, and sadly this weapon is used to destroy Prometheus. To me Prometheus was somewhat part of the cast. And ya know they spend all these years trying to make the ship perfect and all that work gone. But it was a matter of time before the SGC lost one of their own ships. Also Prometheus helped seperate SG-1 from Atlantis-Daedalus.
It was sad to see Col. Pendergast's expression before going down with his ship, but he did it to save his crew and that makes him a hero.
It was a hard, sad, serious episode to watch and it just shows you how things don't always go the way you want them too.