This is my second time playing VTMB, and I am using the latest Unofficial Patch, and when I kill Lu Fang in the quest for the two old hitmen in Chinatown he does not drop a key. This also happened in the previous patch but I was able to get it by restarting the game. But this time I did not get the key at all. Lu Fang is the one in the Golden Dragon restaurant in Chinatown.

When I kill the other one outside he drops the key and I am able to complete the quest.

That may be how it's supposed to work but, I really don't know since I generally get them to be friends again to complete the quest.
What I mean by "supposed to" is that I think the key drop is triggered by the order you question them so the game may have expected you to kill the second guy rather then the first.

Did you talked to the drunk (before killing him)? That may have changed the trigger to the pedophile. I'm thinking that if you didn't kill him prior to saying something like you'd go after the other guy that it may have changed it. Or, it could just be a random bug. Doesn't matter since you finished the task, though.

PS - the drunk is one of my very favorite characters in the game - many of his lines are just plain hilarious. I especially like his lines with a Nossie female.