I can't see slavers as morally grey. Plus CL seems purely motivated by power, now if it was an Ismael Ashur deal, MAYBE. But slavers+power hunger= I dont see the 'grey' there.
Maybe their ultimate goal is better off for everyone?
Maybe by collecting all humans into his army he forces everyone through fear to align themselves to them.
And by that they can restore peace to the wasteland by force instead of trying to sweet-talk to a raider or come to a peaceful solution.
So that once Caesar controls it all he dictates over the people, and through fear, execution and torture he will rebuild the world.
The actions they take will be considered evil for the time, but if he were to rebuild the world and unite the people under one government.
Then wouldn't Caesar have been the good guy in the end?
Sure a lot of people died but they actually restored the world from its former chaos and anarchy.
That's my speculation of it.
House just seems like a greedy selfish [censored] IMO.
And NCR is clearly falling apart IMO.