Strange Otis' sister didn't...

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Becky would have seen the VHS tape playing if she did not have to go the bathroom after she and Henry got back from having their steak dinner, Henry came into the living room and saw that Otis had been watching the rape video and passed out because he was drunk and he looked pissed off and disgusted because of it and because Otis was careless enough to watch it knowing that Becky could come home and walk in on him watching it and find out about what they both had done.

I think that this is another reason why Henry was stressed out and needed a cigarette because it calmed his nerves and he knew that he had to talk to Otis about it, then he returns home and finds Otis raping and assaulting Becky and i think that Henry liked Becky as a person on some level and didnt want Otis to harm her at all, he does protect her when Otis grabbed her earlier on and makes him promise not to do it again, so he protects her here too and gets rid of Otis as a liability, but because he was a serial killer and Becky saw what he did he needed to murder her and get rid of her too, because Becky could call the police and turn Henry in, even though Otis raped her and attacked her and tried to stab Henry in the throat and she stabbed Otis in the eye to protect Henry, Becky was upset over her brothers death and wanted to call the police and later in the car she looked like she did not totally trust Henry anymore which makes her dangerous to have around.

So Otis watching that movie too much caused everything to unravel, Henry knew that it could happen and was angry with Otis because of it, Henry didnt want to go to prison and Becky finding that tape and watching it would call the police and ruin everything for Henry very quickly.