Continuity mistake: When O-ren chops of Boss Tanaka's head, the head is going in the direction of the boss to the left of Boss Tanaka. It is already on its way down and at eye level before the shot changes. The next shot we see the reaction of that boss and the other bosses but the head is no where in the shot. The head then reappears in the next shot and lands in front of him. (00:02:55)
Continuity mistake: When Black Mamba goes into the house in Pasadena, California, to the bottom left of the slide is a ball which is not there when she leaves. (00:05:25 - 00:15:15)
Continuity mistake: During the fight at Venita's house, she overturns a large shelving unit onto the Bride. A shot from the floor (behind the Bride's head) shows nothing on the floor to the right. As the shelving begins to fall, a basket of magazines suddenly appears and the left edge of the unit is seen falling to the outside of the basket. A moment later Venita bolts for the kitchen and the shot cuts back to the Bride. The basket is now outside the edge of the unit (and some of the shelves themselves have disappeared as well). (00:06:40)
Continuity mistake: In the fight scene with Vernita Green when she and the Bride are staring each other down as the school bus pulls up Vernita is holding the knife horizontally with her index finger along the top. The angle of the scene changes and she is holding the knife in the opposite way. When the scene changes back her finger is along the top again. (00:07:10)
Continuity mistake: When Vernita Green and the Bride are talking to Nikki, Vernita has no blood on her right breast, but when they are talking in the kitchen, there is suddenly blood. (00:08:10 - 00:09:55)
Continuity mistake: When The Bride and Vernita are chatting in the kitchen after their knife fight, the sweat on The Bride's shirt changes. It is a small portion at the neckline of the shirt to covering most of the front of the shirt, to back to a smaller portion. (00:11:00)
Continuity mistake: When Vernita Green takes her daughter's picture from the notice board in the kitchen she holds it at the top left corner when the camera is behind her. The shot changes to in front of her and she holds it by the bottom left corner, the shot moves behind her again and she holds it by the top left corner. (00:11:15)
Continuity mistake: When The Bride and Vernita fight and during their kitchen chat, Vernita's zipper is in the black-band portion of her sweater. After the knife penetrates Vernita, the zipper is now higher up into the white-band portion of the sweater just above the knife. (00:12:30)
Continuity mistake: The Bride kills Vernita in the kitchen then pulls out the knife. When Vernita's daughter enters the kitchen there is a front shot of The Bride holding the knife behind her, hiding it from the child. The camera switches to a reverse angle and The Bride is holding the knife in front of her, in full view of the child. (00:13:30)
Continuity mistake: After Black Mamba leaves Vernita's house, she looks at the "Death List Five" notebook page and crosses off Vernita's name. The handwriting on this page (sloppy) differs greatly from that of the list written on the plane (neat) after killing O-Ren Ishii, which is supposed to be the original list. Even if she re-wrote the list, the handwriting wouldn't have changed THAT much. It is a different person's handwriting and not just a difference between sanity and madness. (00:15:40 - 01:41:50)
Continuity mistake: As Earl McGraw tells his Son Number One the massacre at Two Pines house of worship must have been "the work of a salty dog," he gingerly moves in for a closer look at the carnage. Even though the side shot maintains a close view on the upper half of the body, the posture of his movements indicates that he starts stepping forward with his right leg. Yet in the immediate next shot from behind, it is with his left leg that he makes his advance. (00:18:30)
Continuity mistake: When Elle Driver carries the tray with the syringe on it, to The Bride's room, the syringe needle is exposed. But as Elle is preparing the injection, she removes a cap from the syringe. (00:20:35)
Continuity mistake: In the scene where The Bride wakes up, a mosquito is shown landing on her. The first shot of the mosquito it shows it landing with all legs. Then in the close-up shot sequence, you see the mosquito is missing a front left leg. (00:24:20)
Factual error: When lying in the hospital, The Bride's heartbeat (the "beep") doesn't match 69 (per minute) which is on the monitor and also doesn't match the visible flow in the artery in her neck (a few seconds later). (00:24:50)
Continuity mistake: When The Bride is wheeling along in the wheelchair out from the elvator, as she passes the corner into the parking lot, you can see a black car, a white car, a green car and a big red jeep parked in a row. Then when she passes the cars, the red jeep is gone. (00:31:05)
Factual error: In the beginning of the anime sequence we see O-Ren's father from behind, along with his Army rank on his left shoulder. The rank in the movie has two chevrons on top (the upside down "V"s), three "rockers" on the bottom (the bowl shapes) and a star in the middle. The real Army rank of Sergeant Major has three chevrons on top - no rank except corporal has only two chevrons, and corporals have no rockers or other insignia. (00:35:15)
Continuity mistake: In the anime sequence after O-Ren Ishii has drawn the sword out of Boss Matsumoto, the blood is splashing out of him, and up on her face. But the amount of blood on her face keeps changing. (00:40:00)
Factual error: Near the end of the anime sequence when O-Ren murders the crime boss, there's a shot of the wall/ceiling behind her as she pulls the sword out of his stomach and you see the outline of O-Ren in the blood on the wall. In the next shot of O-Ren's face and body she only has minimal amounts of blood on her. Considering there was enough blood spraying out that you can see her outline on the wall behind her, shouldn't the front of her be covered in the blood as well? (00:40:05)
Other mistake: In the anime scene, after the two men empty their magazines onto the back of Matsumoto, from under the bed, O-Ren shoots the leg of the guy wearing brown, then blasts his head. Then she fires a third shot that hits a brown leg, but the guy wearing blue falls instead. (00:41:55 - 00:42:40)
Continuity mistake: When "The Bride" is first in Okinawa and talking in the bar to the sword maker, look at his headband. The black and white stripes change from shot to shot, obviously showing how each time the material was twisted by the costumier before shooting. (00:44:15)
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.