The problem is then that we are dealing with fanasty, a world where Evil does exist, it can be objectively defined and is personified. Here, the taboo of evil is pretty much broken, a Good guy may list a thousand reasons why the Bid Bad is Evil, and the Big Bad will admit to being Evil but reply "So what? I am Evil, but my side has been oppressed by the Good Knight Templars for so long, and my side will eventually trimpuh over your side."
People would self-identify as Evil in this world, they would just see themselves as being 'right' to join up with Evil and sympathize with the grievances of Evil and hate Good. Prehaps one of these people might join up with Good after their family was killed off by Good guys.
Then we are in complete agreement.
It must be accepted, however that the individual taking the evil act does not believe the act to be evil themselves according to their own belief system.
Just because the factions are called Good and Evil doesn't mean that the factions aren't just factions. If you sympathize with and share the grievances of one group, you have concluded rationally that this group is the morally superior group according to your own personal belief system.
Alignment IS attached to the person, because a belief system is attached to the person, but no given individual believes their actions to be evil. To take your example from before, the Group of Heroes might be seen by the majority as the last, best hope for humanity, paragons of virtue and justice, but to the other side, they're Those Fuckers who killed my family. The ones that blew up a power plant and caused hundreds to die on their hospital beds (FFVII, and others). The ones who slaughtered countless young soldiers standing up for their country (Every RPG Ever including Table Top). The ones who caused the total ecological meltdown of an entire planet and responsible for the murder of countless millions if not billions of... Ewoks. (The Endor Holocaust).
My point is that actions can be rationalized. Even in your own example
... My side has been oppressed by the Good Knight Templars for so long,
The Big Bad there has rationalized, and in his belief system has determined that the Good Knight Templars are the bad guys.