Welcome to the large, exclusive club of players who have offed that rat too soon! (I did in my first game too!)
The easiest solution is to load an older save in which he is still alive and play through again now that you are wiser.
The other alternative is to hack the dagger. There is a thread on the board which explains how to do that and includes the code for the Assassin's Dagger.
IF you are wary of doing that yourself, I can do it for you BUT I need you to not only zip up your game file but use WINSPLIT to break it into smaller pieces (5 is nice) and email each separately along with a batch file to put them back together again. My email does not like large attachments. [img]smile.gif[/img]
Wyv