I see orcs a little differently than anyone else has mentioned. Not that I'm necessarily right, I'm probably just weird.
Orcs are described first as originally Elves, twisted and mutilated by Morgoth. Later we see the Uruk-Hai being bred from men, but the idea is the same. They have they same origins as the other "children of Eru". I cannot reconcile this with them being inherently evil or having no souls.
We ask why orcs are used as "cannon fodder". I do not believe that view is "Eru-given", but a human and Elvish perspective on a race they do not fully understand. Humans and Elves may be in the wrong for killing orcs without mercy. This is not the same as saying orcs are inherently evil. The only reason we believe that is because we read the stories as told by men, Elves, and hobbits, and assume that this is the correct perspective.
What if the stories were told by orcs? An abused race, set up for evil from the beginning, yet we see that they do have some semblance of culture. They lived in small clans in different areas, just like other races. But unlike other races, they had been bred to live in the dark and shun sunlike. What grows in the darkness of deep caves that they could eat and drink? Nothing, which rules out their ability to farm or grow crops like other civilisations. This, combined with Morgoth's imbedding their genes with a tendency toward violence and mistrust of others (which surely he did) created a race of savages who relied on hunting, murder and cannibalism for survival.
Even if an orc was taken out of their own environment and put in another, they probably could not adapt entirely to a life of non-violence. I believe they were genetically altered by Morgoth. Imagine being one of the first Elves kidnapped by Morgoth at the beginning. You are taken into darkness where you spend ages (literally) in pain from torture, as well as watching your beautiful shape gradually become hideous and ugly from the Dark Lord's genetic experiments. This pain, anger, and eventually hatred becomes inherent to the nature of the orc and all of its descendants.
However, that still is not saying these changes are irreversable. If it was possible for Morgoth to do this damage, couldn't it have been possible for the other Valar to repair it? But since orcs do not have contact with other Valar (and those that did mercilessly killed them off instead), their possible redemption is never realized.