The fact that both Alicent and Сriston were at the age when their personalities were being formed, but found themselves in a situation where they had to deny a part of themselves (Alicent ceases to be a Hightower, Сriston is fighting against the Dornishmen). And they don't know how to define themselves except through the system that has subjected them to suffering.
And they both come to the point where they want to be good people, but that doesn't save them, it doesn't save anyone, because they keep defining themselves through what the system tells them, and the system is lying.
Even if you are a good queen and wife, it will not undo either your suffering or the suffering of other people, because the monarchy is built on the oppression of others, and a marriage in which a wife serves her husband can easily deprive you of even those rights that are due to you under this unfair law.
And even if you are a good knight, it will not undo either your suffering or the suffering of other people, because being a knight is being a murderer, and war always destroys, and even if you have risen despite your origin, you are still dependent on someone else's whim.
You get angry, and your anger hurts others, and you hate yourself, but you don't have any ways to be kinder, because you don't know how.
And then you find each other. It makes you feel better. Enough to deceive yourself that you are good people.