While a part of this is true, we also need to realize that if we are not to look at these characters from modern perspectives, we need to use the perspective of the society in which they reside, and they are evil even by those standards. Cersei and Jaime have an incestuous relationship, Jaime is a regicide and an oathbreaker, Cersei hopes to commit fratricide, and so on.
Most people would laugh away some of these reasons to call them evil or even wrong, though their own world would not hesitate in branding them as evil and accursed.

The total bullshit-ness of this argument is astounding. Cersei and Jaime deliberately disregarded their own safety and that of their children when the spent years f*cking around in strange castles. They know quite well that Robert would kill them and their kids the moment they were discovered. Yet, having sex was more important?
By your argument, it is completely normal parental behavior to risk your child's life by having forbidden sex, and then murder another innocent child when you're discovered! Arrant nonsense.