Perhaps, but I'm not convinced the Three Oaths would have stopped that. Look, we see Joline, Teslyn and Edesina hurl fireballs at an oncoming army of Seanchan. They could have just woven a wall of air to protect themselves from their arrows. But they chose to kill them anyway.

Whether the Three Oaths would have stopped this or not, though, within the combat situation they were in, Egwene did not violate any basic laws of combat by killing the sul'dam. They hadn't voluntarily disarmed, and certainly hadn't offered anything like a surrender, and were quite unlikely to anyway.

Egwene already overdrew before she killed the Sharans. She was perfectly matched with Taim, and had to overdraw to overcome his Balefire. This was a done deal. She was either going to release the source, and be burned out, and completely in no position to stop the Sharans or deal with the new hole in the Pattern, or else overdraw even more and do both those things, risking death. She chose the latter, which is perfectly fair, and was the sensible and right thing to do anyway.