You're so close...
The point of labeling things "evil" is to justify killing things and taking thier stuff. It's the ultimate expression of vigilante justice, conquest of the righteous and night making right.
You kill orcs because they are irredeemably evil. They worship Heathen gods, have barbaric practices and cannot be taught right from wrong. You kill cultists of Orcus or Tiamat because their beliefs are different from yours and that causes them to do down the road of violence. You kill owlbears attacking farmers who are settling the wild lands and bringing civilization to the frontier.
D&D always takes the side of the conqueror, the vigilante, or settler. It used alignment as a tool i justify the violence in fiction, but D&D will do just fine justifying putting orcs, cultists and other "monsters" to the sword without it. It's a game based on violence against others and as long as you believe you can morally justify the violence and theft, it will work.
Which is why my ultimate takeaway from any sort of real-world morality grafted onto gaming is that you accept the premise your foes actually are evil and thus you can participate in the violence OR you believe all conflict comes from a place of conflicting interests and mediation and resolution should be the primary method of resolution, with violence as a sad last resort.
Because if the orcs aren't Evil, they're just in your way...