
Revealing mistake: When the swordmaker writes "Bill" on the window, take a close look. The window has been wiped before (as happens at the end of the scene) and "re-steamed" - you can see the outlines of the wiping from earlier takes, with a clear contrast between that pane and the others. (00:52:45)
Answer: From what I understand the reason for the lights being off is the same reason for why the previous scene was done in black and white; to decrease the amount of 'graphic violence' in the movie in an attempt to keep an 'R' rating. I would assume that they had him shut off the lights for that scene as just another method to accomplish that task.
I believe the original question was asking why was it done within the context of the film (i.e. why did the character shut off the lights) not why was it done in reality. My best guess is that the manager switched off the lights thinking the 88 had a better chance of killing the bride if she couldn't see. True, they couldn't see either but there were so many of them one could possibly have gotten to her.