okay, here's what you need to do to kill Marburg in the Rome Mission.
1 - Have a reputation of -7 or less with Marburg
2 - Use Suave responses to anger him (this is also the ebst way to get a low rep with him over the course of the game).
3 - You must ahve used Suave responses more than Professional and Aggressive ones over the course of the game so far (its basically a reward for being suave).
4 - Maybe need at least an 83% completion on Marburg's dossier (getting 100% means you bought the dossier info on him before meeting him and therefore would not be able to get a -7 opinion from him, unless you edit the savegame).
Speaking of the savegame editor, what's the 10th skill - "Infiltration"?
I can't confirm this, but I think it's a skill that the game calculates behind the scenes based on your play style and how stealthy it is. I noticed a couple parts in the game where you sometimes get a special extra dialog choice 'infiltrate' that will allow you to accomplish two tasks in a mission where it's usually an either/or situation. Sadly it doesn't show up for the Rome - Madison scenario.