I think all you have to do after that is go outside to the front of the mansion and talk to Mandy. That should trigger the completion of the quest.
If that isn't an option or Mandy isn't outside, then I think what you need to do is go back into the room with the killers, equip the knife, and then attack Abigail. After that, attempt to leave the room and a mini cutscene should start. You need to click through the dialogue until it finishes, if you click away before the dialogue is complete it won't let you continue to the next step of the quest.
Hope that helps!
- Last edited on 10-Aug-2019 03:27:38 by ZeeZee
After you "kill" her and you attempt to leave the room, a mini cutscene will play where she gets up and attempts to kill you. At this point Mandy should save you and the cutscene will end. After the end of the cutscene, Mandy will be waiting outside at the front of the mansion. Once you talk to her the quest should finish.
As far as what is going on, I think it's just the way the quest (and most quests) is designed. The next step of the quest is unlocked after you clicked through the dialogue. If you don't click through the dialogue, often times the quest won't proceed to the next step.