Remember, this is a game. As such, it has a goal, and certain peices are required to realize that goal. If you are able to achieve (or just do) the unexpected and kill him on your first meeting, the game accounts for it, makes him a liar (which he already is) and presents you with what you need to continue.Or he is just lying for some reason, like for the sake of lying ... for him it is perhaps good practice.
I have thought on this aspect of the game ... not lying, but what happens when character X gets killed before he has a chance to do Y?
If I were to write me own game, each of these things (like the acquisition of the real AD) would become an onus (something to do) or token (a physical object of importance). Whenever a character died bearing an onus, or carrying a token, another character would be chosen to bear it.
Say you kill Don and he didn't have the AD yet. It was supposed to get to him somehow ... so now the "delivery boy" keeps it. Or say Don ends up dying with it, but not at your hands ... natural causes. Then Milano would find it ... pick up the token.
It would be a lot more work, and would probably require some kind of factory (in Java terms) to create replacement characters. Otherwise, you could just kill everyone but Vi, anad she would end up with all the onus and tokens ... then they woudl be easy to find (right inside Phoonzang's statue 
Tools for Wizardry(r) 7