You made this thread because, as you stated, you felt she was acting out of character compared to her past, so this is rubbish. If she had been evil from the start, you would not have come up with the idea that she was being controlled.
I mean, your very next sentence contradicts your first:
Was her evil consistent prior to the Cataclysm or was it not? Decide!
Like, let me grant you some vindication and make this theory:
Sylvanas is controlled by Helya. We know her soul is alive somewhere. She was able to do so through the pact they made. She influences Sylvanas to make her cruel descisions, which is why none of her arguments make sense. This is all a roundabout way for Helya to exact revenge on Odyn by killing Azertoh through the blood war.
There, in two minutes I made a theory that is better than yours, and has more evidence to support it. Now Sylvanas is less evil. Everyone is happy. “The Hour of her third death” refers to Helya, whom we will kill when we confront Sylvanas. <-This is actually true, it refers to Helya.
Because we killed it… To the Old Gods, the concepts of death and sleep are synonymous. They are parasites that feed of Azeroth. For them to be truly “killed” or rather destroyed as to be rendered harmless, they must be torn out of Azeroth and crushed. This tearing is what caused the damages. Killing them/putting them to sleep like we did renders them inactive.