Lanefan
Victoria Rules
Undead fit the bill for sure, though using them gets stale pretty quick particularly at low levels; never mind that some classes do very well against undead and others not so much (particularly in old-school games), so constantly using undead as the foe is going to play some merry hell with your long-term class balance.To answer your question about evil forces that players can face with little qualms - here noting that this would not be the first (nor I imagine the last) time that people have given you acceptable answers for you to discard - undead, aberrations, abominations, and the like are still available even if one does not want to use fiends.
Aberrations, abominations and fiends also fit the bill but only for mid-to-high level parties - there's still a hole at low level.
Couple that with a desire to have opponents be vaguely relatable - as in: made of something resembling normal flesh and blood, native (or close enough) to the PCs' home world, and humanoid enough to count as a 'person' for purposes of being affected by the varions 'xxxx Person' spells (Hold, Charm and Dominate are three such) - and we're back to, in effect, looking to replace orcs and goblins and so forth with similar-but-different creatures* for use as go-to low-level foes.
* - or, back to just using orcs and goblins and suchlike and having done with it.
Lizardpeople are one possibility, troglodytes another; though in both cases their preferred swampy habitat rather severely limits their spread. Easy enough to refluff kobolds as a smaller variant on lizardpeople much like Hobbits are pretty much smaller variants of Humans, to give some more variety.