To put what Egwene is thinking in perspective, she has never fought to the death against a Forsaken - she doesn't really know that Moghedien is a coward, nor does she know Moghedien's combat capabilities. Moghedien is actually quite undeveloped in that area - she could have destroyed Nynaeve in a flash back in the Panarch's Palace, but instead she went for the least accurate means of defeating another channeler possible. This indicates something. Moghedien could gauge Nynaeve's strength; she already had earlier in the book. Instead of taking Nynaeve out in a surprise attack, she was beaten by her own lack of fighting capability. She didn't want to fight to the death - she wanted to capture someone in her web, and she in her turn, was captured. I think Egwene herself is quite capable of destroying Moghedien because she is not a coward, and has learned from her mistake - some threats must be killed when you have the chance, rather than captured or neutralized, because it isn't a game.