Levistus's_Leviathan
5e Freelancer
Tolkien's Orcs have language, and different personalities and are intelligent enough to make weapons/armor and other crafts. They're people. They have some kind of free will.I think I laid it out pretty clearly that they weren't free willed in the OP. If by "people" you mean flesh and blood mortal creatures, there's no reason not to. They are more like velociraptors in that sense, but more frightening because they think and speak and might just want to chop off your legs for a snack since you aren't using them anyway.
Then use magical wolves or some other type of unintelligent wild predator. Or something like Warforged. Or make the robots biological robots (like @Oofta's version, where they're clones programmed to follow certain orders). Or give them some other justification for being hostile than just "they're born evil". Maybe they're like Thri-Kreen or Lizardfolk and enjoy eating the flesh of one of the other races. Or need to fight for some reason, like 40k's Orks. Or it's just a specific culture/religion/nation in the species that makes them "evil".As to the "why not skeletons/robots/demons" question: I think, world building wise, they are scarier as "alive" but I can appreciate that some folks prefer not to draw some arbitrary line between "people" and "servitor species."
There is no justification for having an always-evil race of mooks (that are people) be used as mooks other than tradition and nostalgia. There are better solutions. There are workarounds for using them other than making them people.