-Would it be evil to slaughter them, given that they are, after all, noncombatants, even if they are orcs?
Yes. Because Good "respects life and values the dignity of sentient beings". Even in D&D worlds, the most common "war atrocity" soken of is when an army puts everyone in a conquered city to the torch.
-Would it be evil to let them live, if you view orcs and other humanoids as inherently evil?
No. If you view orcs as "inherently evil", you're misguided, since orcs aren't "inherently evil". Demons, Devils and Chromatic dragons (among others) are "inherently evil'.
-Do humanoids, IYC, have the same rights and deserve the same consideration as demihumans and humans?
There's no distinction since 3e between "humanoid" and "demihuman".
-What if your character is a dwarf, who may well view these creatures as inherently evil? Could it be an act of Good, from his point of view, to destroy them where they stand, so they cannot breed more of their kind?
In the D&D alignment system, there's no such thing as "Good, from his point of view". Good is Good, Evil is Evil. They're absolutes.
IMC, during the Forge of Fury, the party came across a tribe of troglodytes (expanded the trogs in the second level of the module). They killed the warriors and the evil priest who led them, then allowed the females to leave with their young, with a promise to never return.