I had the same problem. first i made a deal with the slavers. but then killed them at Hannibal's place. then started the quest with Hannibal by getting the picture from the museum. this definitely messed up some of the quest variables because, after i completed all of the objectives, the slaves just hung around the metro station. All of my quest stages showed complete but the slaves were waiting for me to kill all of the slavers. i read on other forums that i could just resurrect on and kill them again. unfortunately i couldn't resurrect the slavers because they were no longer there. my solution was to enter the following command line entry: set MS06.SlaversDead TO 99.00. Using the GECK, I found that this was the condition that the quest was looking for, However, I could not find where this variable get set to 99. Once i entered the command, i went to talk to Hannibal and he told me he was grateful for my help and gave me the dart gun schematics. then all the slavers proceed to the memorial and the quest was complete. I spent a lot of time trying to find a solution for this so i thought i would share what i found because it worked for me. i hope it works for anybody else with the same problem.
