Cast "Speak with Dead" on the victim and ask them 1) who killed them and 2) why?
The guy just died. Why not ressurect him? Surely there is some non profit in Baldur's Gate that could help out? Murder in Forgotten Realms is likely to be a minor misdemeanor.
Resurrection is a really expensive process. I'm sure there's clerics in Baldur's Gate with enough power to do it, but I doubt there's some kindly soul rich enough to provide the massive, expensive diamonds required for the ceremony. Especially since the city sounds like a pretty rough, cutthroat place.
