Earlier as Momon he asked them all why they would do it. They all said it was for money. If a few said they were being forced to and they don't want to loot/disrespect a tomb then I think he would have let them live. Lke those elves/half elf slaves. They weren't shown to be killed so I think they were instead recruited. I mean he even gave them a chance to line their pockets with golds when they found a few tombs stuffed with it. That would have been in the end of it. But they got greedy which led to their deaths.
The only thing he cares for is Nazarick and it's inhabitants. The rest can die for all he cares. Sure he does not want to kill innocents and he himself does not. But the moment you touch Nazarick you are no longer innocent.