His niece asks you to spare him, ok try to be the good guy...I destroyed both glasses then used sleeping darts on the campbell and curnow. Left curnow alone and then branded curnow as an exile...but if I didn't kill curnow why does his niece think I did? anyone else have this issue?

You have to make sure Curnow's unconcious body is taken to a safe place away from the action. Otherwise you left him at the scene of the crime where the High Overseer was killed/disappeared. Remember, he was privately meeting with Campbell, so congratulations, you just left him there to take the heat/be executed by the Lord Regent. So Curnow's neice is correct - you inadvertedly killed the guy.

Basically, make sure Curnow is taken outside the building, far away from the action, and hidden in the "Safe Spot" the game points you to, before you leave the area.

NOOOO...I didn't knowwwww!...but I can replay the mission right? right? how do I fix this, I must repent!

There's an additional option. Just don't do anything to fight him and he'll get away on his own.

Yeah, this works. I was sneaking along the upper ledge and entered the meeting room via the small window above the door. The opposite door flew open instantly to reveal Curnow and Cunningham arguing, so I shot the Overseer with a sleep dart. Curnow proceeded to take off running, and about a minute later I got the bodyguard achievement without ever leaving the window ledge.