I think you can't explain away being evil as that. Actions are what determine it. Or determined to do something. Which could extend to how you influence others to act as well. So far he's really tried hard to play the good guy.
Evil also means to have next to no morals, or even take pleasure in others misery.
He's a being of Evil, that is supposed to be Evil in nature, but with his free will he chooses instead to be kind and caring. But at the same time he tries to embrace evil nature and explain away like he did, it doesn't make sense, as he rejects acting in immoral ways or harming others.
If anything it's disingenuous to explain it like he did, unless he really thinks that which otherwise makes it stupid.
Essentially, being evil is being selfish (to the point where it's out of the norm--for ex.: "you refused to shelter our relatives, even though we had the means, because you didn't want the inconvenience...". Thing is, it's not a either or thing, but partial. Nobody's 100% good/evil ("shades of gray"). When someone's actions are mostly damaging to others, they're considered evil (bad). And someone can be good to some people and bad to others and such, which complicates things.
Normally, good actions are altruistic ("give"), and evil/bad actions are selfish ("take"), because we depend on resources since we are struggling for survival.
He did make an evil/bad action, though, by annihilating those soldiers. He being mostly good doesn't excuse that. It's acceptable because it was self-defense, but
he chose the role of a conqueror, knowing that it'd cause (outstanding) conflict thus loss of life.