leaving them alive was risking them killing inhabitants of the Tower. There is a reason the Seanchan assign military rank to the Sul'dam and Egwene would know that from her personal experience as a damane.
What is that reason? I would be fascinated to see your thoughts on this, since military ignorance is pretty much a general characteristic of most people on these boards. Military ranks has NOTHING to do with combat capabilities, or how dangerous anyone is. Military rank indicates the number of soldiers you can command. Alone in the White Tower, they have no soldiers, and their rank is meaningless. I suppose I should not be surprised at this sort of absurdity, since an inevitable fallback argument of Egwene defenders is her possession of a high rank she did nothing to earn. Rank is NOT an indicator of ANY innate quality, by anyone. Holding military rank does not make you inherently dangerous, nor it is given on the basis of danger or ability. A sul'dam can do nothing without being complete, and Egwene made sure to fix that before murdering them. And again, they can't do jack in bonds of air. Just because it is their "mission" to capture people doesn't mean that they have the ability to do it, or justify murdering them once they are helpless.
These women are trained in military thought and are more dangerous than simply being a leash holder. Leaving one alive in an active fight zone opens yourself up to having a knife on the back or warning the enemy of you position or a dozen other things an active combatant might think up. Sul'dam are meant to be front line combat specialists, I'm certain over the years they learned a thing or two about how to engage when their damane is not around to channel with. These are no innocent women who can't continue. Fight once her main weapon is gone.