It's a slippery slope.
Morgana hates the screams, the smell, the obscene spectacle, and the source is clear. Uther is becoming an easier person to hate, with every pyre that goes up in flames.
Some of them are children, so young that they have not yet been taught how to mean harm upon others. There are no trials, only the word of a man who is the law, and the pronouncement is always the same. Some of them must be innocent, she knows, she's seen too many vindictive neighbours to suspect the intention behind their accusations. Of the guilty, surely there are those that are just subject to things they can't control. Her nightmares get worse, and more unnatural, and Morgana can't help but feel kinship with these sorcerers.
That is when she first believed that Camelot would be better off if Uther were dead.
It doesn't pass. The King abuses his power so often that the thought never has time to fade. It grows.
What is just one man, it whispers, just one man in return for thousands?
It was easier to bear when she was powerless to carry it out. It doesn't remain an idle fantasy.
The thought becomes a duty. She learns she has magic, and she knows their fear and pain more than ever before. She learns she has a claim to that throne and with it, the people of Camelot are hers to protect.
Uther must go, he is a curse on the land, a parasite creating hatred and spreading harm. Arthur might not be a tyrant, but he has listened to their father for too long; Morgana knows she can do better.
She didn't mean to get anyone else involved. The first death was an accident, the second one a reflex. Sacrifices became necessary. She put value on a life, and that number became harder to see and easier to discard.
Somewhere along the line, lives became obstacles.
Uther had fallen and she still had not succeeded. Arthur and Merlin were too powerful. Stealth failed, assassinations were fruitless. She needed help, she couldn't do it alone.
Somehow, it turned into a war.
Her people are dying. His people are dying. She can't tell how many, she can't tell who is winning; in death the bodies all look the same. And she has fallen with them. She has fallen so far that only victory remains. She has to win now. She crossed a line a long time ago, without even knowing it. She has sacrificed too much for it to have all been for nothing.