In terms of what the killer actually wanted. It wouldn't necessarily have to be something tangible. Perhaps it was information the killer wanted written down? An address or some bank details or something to do with "Paul"?
The perplexing aspect is that assuming that what the killer was after was related to AW's work, (eg account details, etc) - why would AW have this information at home - or at least what type of information could be 'useful' to someone from someone in AW's position?
Or was it other information? A task?
If the reason was to obtain something from, or get AW to act in some way would calling at his house be the best way to achieve this?
Could pressure not be exerted in other ways?
Either the killer achieved their goal then killed AW (As far as we know, unless AW told them something or furtively wrote something on or put something in the envelope) - they did not. Did the act of killing AW mean the 'goal' was not achieved? If the goal was important enough to kill for, what happened next? Did the person behind the visit get what they wanted some other way?
Or was AW 'silenced'?