Originally Posted by
nothingsjim
This is from Wowpedia, and frankly, I don't know how much of it is canon, but I do think it make a bit of sense.
Upon first being raised, a Forsaken does not have free will. It takes more concentrated magic to release an undead's mind. Free will is one of the cornerstones of Forsaken culture, with the great capacity for both good and evil that it entails. However, some undead, especially those who die in combat or under extreme stress and are raised soon after, enter into a violent, frenzied state. Undead in this state are easily manipulated and their rage is often directed at the foes of those who raised them.
Sira and Delaryn are freshly risen, and both died in combat, so it makes sense they enter a "Violent, frenzied state", and Delaryn staring at her hand after the battle is over feels like a "omg what have I done" feeling to me.
I sure hope this is the reason, we don't need to tag Sylvanas with more #EVIL tbh.