I just started playing tonight and decided I wanted to mark Campbell with the Heretic's Brand and keep Curnow alive. So I smashed both glasses and hid in the corner behind the changing thing. I shot Campbell with the sleeper dart which caused Curnow to run to the alarm. I couldn't get him with a sleeper dart before he hit the alarm, which I immediately turned off and knocked out the one guard who came running (I had killed the rest on that floor). I proceeded to mark Campbell and I got check marks for the non-lethal method and for keeping Curnow alive.

...then I get back to base and I talked to Callista Curnow, who says something along the lines of, "My uncle is dead, isn't he? He never came back. I shouldn't have expected anything else." Wait, what? Did Campbell tell the guards to kill him? Maybe they thought Curnow marked Campbell. I dunno. I'm just confused. Anyone else have this happen?

Well, did you knock him out? If yes, you need to store his body at the "save location" the game is sending you to.

You have to hide his unconscious body or else they will kill him. My advice is to tranquilize him, grab his body and exit through the window, then hide him in a trash bin.