My thoughts on why the actions of Sylvanas actually fit her character.
I think a lot of people don't really understand what Sylvanas and the undead actually are. Hopefully this will help a lot of people understand why she has done these actions and how they actually are perfectly in line with her character. I mean her actions fit what she is perfectly.
One thing is this comes back to the curse of undeath and the downsides of it and what it instilled in Sylvanas when Arthas raised her. Remember all the undead still feel the addiction originally instilled when raised.
The more powerful the undead type the worse it was such as the San'layn need for blood, Death Knights need to inflict pain, the need for zombies to feed on the living. Banshee's feel an intense hatred and jealousy of all life. The weaker undead can somewhat supress it, but the stronger undead like the death knights and San'layn can't, they have to give in or they feel intense pain until they go mad.
Think about how powerful the urge for Sylvanas who is one of the strongest undead in existence to kill every living being she sees is. She has been suppressing the urge for so long to try to lead the horde. I think Sylvanas has been fighting against her banshee nature for too long and it is tearing her up inside.
Banshee's have an irrational hatred of life and the world tree is a beacon symbolizing life. Her true nature as a banshee caused her to destroy the world tree. Even the line in this cinematic about her becoming an enemy of all life fits with this. She is losing the fight against her banshee nature, even her looks are becoming more banshee like in the recent cinematics.
I am wondering if this new light based form of undead might also factor into this. Undead are beings of shadow/necromantic magic, that is why channeling the light gives them pain. There is balance in the light and dark in WoW, The undead are ruled by the darker aspects at the moment and suffer because of it.
They need light to bring a balance. I have a big feeling that Calia and Saa'ra(a naaru that has experienced both the light and the dark) might play a big role in helping Sylvanas. By bring a balance to the undead they might be able to end the crippling addiction part of the curse they feel.