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).