Because it's impossible to have a different opinion from yours? No, I don't think so, Max. I know what "evil" means...
This is self-contradictory, as stated. You implicitly assert that we can have differing opinions - that implies that there is more than one meaning. It follows that you (or Max, or I) don't know what evil means. You at best know what you feel is evil.
The difference between knowledge and personal opinion should be carefully maintained in this context, especially after pushing back on someone else for making the same rhetorical error.
I know I don't think killing someone in self defense - even if choosing to defend yourself with more lethal means than they attack you with - is evil.
The folks around you, broadly speaking, take it that it is okay to meet undeserved force with equal of lesser force. The culture you are in, again, as a broad generalization, does not believe that it is okay to take a life if yours is not in danger. If someone whacks you with a rolled up newspaper, drawing a knife is not justified. If you are looking at a broken nose or rib, or arm, and the other guy ends up dead, you are in the wrong, even if the other guy started it. If you go too far, you step out of the realm of justifiable self-defense, and into your own retaliatory assault.
This is especially true in fistfights. Humans, without a whole lot of training that the vast majority of us don't have, are very bad at killing each other bare handed. One-on-one, until someone picks up a weapon, it is unlikely that anyone's going to end up dead. Drawing a deadly weapon in such a situation is going to typically be seen as escalating aggression, not self-defense.
Now, that's not to say you cannot have a different opinion. But, as soon as you call it a differing opinion, you must also recognize that what you call evil will be meaningless when considering what the world will call your actions.
This allows us to bring this back around to the context of fiction and gaming. For those purposes, there are two kinds of villain - the sociopathic or nacissistic ones who just don't care what others think, and those who really honestly feel they are doing "the right thing", but go too far down the road of the ends justifying the means. In either case, the villain is not defined by what they think of their own actions, but by what *others* think of their actions.