I'm claiming that there's a comparison to be made between two groups of people who justify their actions with "the ends justify the means".
If you convince yourself that what you're doing is right, that it's for the "greater good", and that it'll truly make the world a better place, then you can do anything. Even kill.
What, when where, who and how are not insignificant details here. You don't just blanket apply generic statements that are not even remotely comparable to the same options available for different situations. Yes, it was " the greater good" to kill the Nazis to stop them from continuing their genocide. Yes, it was " the greater good" for the Native Americans to kill the Spanish who were carrying out a genocide against them and drive them back into Mexico to prevent them from being " exterminated". Sometimes, yes People are forced to defend themselves with lethal force, that does not mean this applies to everything or anything. The details actually matter here.
Of course people convince themselves of all sorts of bad things and think they are "right" , THAT is exactly why the Nazis were committing genocide in the first place. THAT is why the Spanish thought they were in the right to tear nursing infants from their mother's and feed them to dogs as well, but you see, the details are important here as to determine what was and was not actually "right or wrong", and no, The winner does not automatically get to determine what is and is not the truth here, the truth still comes out in the end, otherwise we would not now still know what the Spanish actually did in the first place.