Who were the Nazgul?

Originally Posted by Raynor

Nothing in that quote shows a hostile intent from the balrog, so there is no need for Mordor to fear him. If Sauron thought he was a threat somehow, he would have ordered the implosion of Moria on his head.

Sauron may have thought of the Balrog as the equivalent of Shelob in Cirith Ungol, as a guardian of a domain that belonged to Mordor but nevertheless a co-existance was inadvertantly made, since it prevented the enemy from seizing it.