When you assassinate Campbell and exit into the backyard, one of the overseers is sick and is with two other of his fellow brethren. I played through this mission three times, the first two where he protested that he wasn't sick and was eventually backstabbed his his friends.

However, this time around, I was peering through the window and the sick overseer was pleading for his friends to kill him so that he could end the plague. Does it have to do with the number of people I've killed? The darker the game, the more corrupt the people are?

When I did the first playthrough (without killing anyone), he begged his comrade to kill him before he became worse and infected the others, whereupon he was given a painless and merciful death.
When I played a second time (this time killing everything in my path), the others were accusing him of hiding the plague he was carrying while he insisted he was fine. The others then cut him down while he begged for mercy.