So if the mc happen to utterly destroy the slayers down the last man what will it be? He would have blood of ten thousends on his hand but all other race would hail him as hero. ...
Unfortunately, you are quite right: History is written by the winners of wars, and the winning warlords are heroes. And winning heroes are mostly considered as "good persons" (in the view of the winners at least).
It's not easy to judge (good and evil), even oneself. And to judge others is mostly a failure. Who knows everything about others?
The example of a piece of paper with an oil spot on in shows: If 2 ppl have contrary points of view, both are right - both speak the truth (at least their personal truth, dependent on their individual position).
Still, although both are right, none of them is necessarily a "good person" because of this.
A "good person" is imo the one who tries to find a compromise, who wants to bring peace to the quarreling parties and to make them understand.
Now Arthur, coming to your example with the slayers: It is not easy from our position now to decide, if it is absolutely necessary to slay ALL slayers of the world to end their evil acts. Or if is enough to defeat their armies and punish or kill the leading responsible ones.
Or if there is any other way, so that there is even not the necessity to defeat any slayer army. (Gandhi did not have any weapons, but he succeeded to throw the english colonial government - highly armed - out of the country).
But slayers are more like romans, much more ruthless than the english of the 20th century. Therefore my guess is that there have to be assassin like underground fighters and undercover mages to kill the slayer imperator and his closest fanatic followers. This would possibly avoid a war with huge losses on both sides.
I like the following example of the movie Avatar really much, and we could learn a lot from it.
Anyone remembers when the MC was in the jungle at night, and he was attacked by these extreme dangerous meat consuming beasts? Neytiri (chiefs doughter) came, killed one beast, made the others flee, and saved the MC's life. Did she hail herself as a hero, a winner, a savior of the good ones?
No, just the contrary: she cried for the killed beast, begged for forgiveness, prayed for it and blessed it. And then full with pain, shouted at MC because HE was the ruthless intruder, making a lot of noise. The beast did just follow his nature. All can live together in balance and with respect to each other.
The remembrance of this emotionally outburst of fury makes me still getting goose bumps.
