I'm working on a little side-quest type storyline for my players while they're between big campaign beats. Earlier in the campaign they came up against a pair of Incubi who fled before they could be killed, so I'd like to bring them back but in a unique way.

The idea I had is that the world they're currently in, humanoids are very antagonistic towards the non-humanoid creatures (think anything in the monster manual, really). However, some of the higher intelligence 'monsters' are actually tired of fighting and just want to be left alone - so they built themselves an underground sanctuary between the surface and underdark where they are trying to live peacefully away from the prying eyes of those pesky humans/elves/dwarves/etc. They'll be led by a cabal of a few of the more intelligent creatures - Mind Flayer, Vampire, etc. Maybe ruled over by an Erinyes.

What I'm sorta looking for is a few short plot hooks to give my players if they're interested in exploring this underground culture; preferably with some sort of moral question for them to have to deal with. Does anyone have any experience of good ideas in this vein I could throw them?

It’s not really a side quest but if the adventurers have a random encounter with a small group of these monsters in a camp and kill them without asking questions, as adventurers normally do, have the young offspring come out of hiding and mourn the death of their parents. Have a few situations throughout the story where you make these creatures seem as human as possible. Get your players to listen in on conversations that the monsters are having, get them to mention how they’re fed up of people always treating them poorly and that they’ve heard of this new place that they could go to. 

So to be clear, you want this cabal of usually monstrous/evil to be non-evil-ish and just want to live their lives in peace. then you want your group to stumble upon them for what gain? What is the actual end goal? Just to show them not all monsters are evil so the next one they go up against and say hi it squishes them?

I could see a plot line that they are looking for something very unusual or information,  but a guy knows a guy, but the guy he knows is really secretive. then if they decided they want to battle it out, the cabal ties them up and kicks them out to get the point across they do not want any trouble. I mean, those denizens need to get some supplies from someplace. maybe they have a sweet tooth or are addicted to milk of all things?