Now ask yourself. Do you consider any group of people to be evil, just because they they belong to that group? Maybe the MAGA crowd or Antifa crowd? Maybe just FOX employees or CNN employees? Maybe some group of rich people or maybe some group of poor people, such as immigrants from some specific country? Maybe Christian or Islamic fanatics?
If there is any such group you tend to see as Evil just because of their group membership, you are already at risk of ending up with the us-vs-them mentality that is the prerequisite of (3).
I freely admit that I'm (at an early stage) of such a state now, as we speak. In the Ukraine vs Russia conflict, I cheer for Ukraine. That means that I don't feel too much compassion for Russian soldiers that die. It is not important enough to me, that I really think that those Russian soldiers are Evil, but it is enough that I've lost most of my empathy for them, as long as they keep fighting.
Whenever some group that you see as Other start to become a threat for some group or some goal that you attach high value to, there is a tendency to start to dehumanize that group of Others. If you think they are Evil, that means the process already came quite far. Right after evil, is to see them as not deserving the protection normally given to humans. Effectively, they become orcs, or virmin, to be exterminated.
This may be hard to comprehend for someone raised in an affluent, peaceful environment. But once there is a really serious conflict of interests with another group, things change. There are a few factors that can massively expedite this process, such as: 1) People that experience hunger/famine seem to often become extremely hostile to competitor groups after. 2) People who experience (relative) powerty tend to have similar, but weaker reactions. (Think, living through the financial crisis in the west). 3) Real hostile actions perpetrated by members of the opposing group. (Ie George Floyd, the 2020 protests, January 6, 9/11, just to take some American ones). More serious ones are war crimes perpetrated by Germans in WW2 or Russians in Ukraine, if you identify with the victims. 4) Real or perceived threats from the opposing group. (Like Russia threatening to use nukes, or (from the Republican perspective) Democrats opening to borders to get more Democrat voters) 5) A subtle feeling of inferiority. The best example is how wealthy Jews were hated in Europe prior ot WW2, and especially in Germany. Especially when the Germans were living through hard times in the 20s and early 30s, the wealth of the Jews at a time when ethnic Germans were poor and starving, was a to the story that Germans told themselves about being a superior race. Only by defining the Jews as Evil, could the Germans retain their sense of dignity. Because the alternative was too painful. Maybe the Jews were in some way superior to the Germans?
But more than anything, I believe the main cause of these kinds of unreconcilable conflicts, is when we think that we are the Good Guys. The only way to maintain that belief, is to always identify our competitors, adversaries or enemies as Evil. To think that we are willing to fight in a conflict vs some other group just to further our own interest can be too much of a challenge to our self-perception of the Good Guys. Much easier if the enemy is Evil, and not really human.
There is an alternative, but it is a difficult one. Imagine that a lion is trying to kill your child or other loved one. You have a gun. What do you do? In principle, you may be an animal preservationist. But for YOU your loved one is (most likely) more valued than some random lion. You don't need to think of the Lion as Evil, and you may even feel sad as you pull the trigger. Such a loss! Maybe even if you miss, and the Lion kills your loved one, you still don't hate it. Maybe you even let the Lion escape with the dead body, as it is already too late to save your loved one.
Imagine you are a soldier in the trenches of Ukraine (on either side) and you are loyal to your side. Instead of seeing the enemy as Evil, you still think of them as just as human as yourself, just with other values and interests. As you kill him, you are sadedned with empathy, but you still kill him, because the things you are fighting for are more important to you than his life. Maybe the guy next to you cheer, as if the person dying is a good thing, and maybe that makes you even more sad inside. You are not a pacifist, you may be the best soldier in the squad, but you are also not driven by hate. When the enemy surrenders, the guy next to you may want to shoot them, but you make sure they are taken prisoner instead.
This approach is much harder than seeing the opponents as Evil. It requires a higher level of maturity. But after the conflict is over, not only do you not face war crime charges, you may be more likely to be able to live with what you have done. The guy next to you may have nightmares about his crimes until the end of his days.