Sorry my english; not a native speaker. (If you see something wrong you can tell me, it helps me improve)
I think it was Aimes.
Otis clearly put something in the champagne and it was not meant to Reese to drink, but yet, nothing happened to her, so was not poison. Could be Xanax for Aimes, or even for Phoebe to chill a little and let Tex alone.
Aimes and Otis are definitely plotting something, you can see they talking with each other before they came with drinks, probably he's informing her that Reese drank the special champagne. They are always pouring drinks to everyone.
Tex tells you that you're the only one who treats him right, but Otis looks very concerned about Tex feelings right before the incident.
Francisco have the murder weapon until he goes to the kitchen with Aimes, and then he disappear completely. He's dead or unconscious, and that's why the fire takes place.
Reese's probably drugged and drunk despite being awake, I don't think she's ok enough to perform a murder.
Both, Aimes and Otis prevent you from calling the police on different occasions.
Both, Tex and Phoebe are with you at, what I think, is the time of the murder, which is when Benjy goes to the bedroom looking for his phone and Aimes says, in a very suspicious way, that she's going to pee. Then you talk with Tex while Phoebe is dancing and hearing you two with her superhuman hearing.
You're with Otis when the power is cut off, but by this time, Benjy's already dead.
If you put out the fire: right after the lights blow, Aimes comes to the kitchen and says that Benjy was killed and she even knows the murder weapon. But I don't think she have enough time to see the corpse and then come to the kitchen.
If you do not put out the fire, she doesn't even show up at the crime scene.
I think she's not heading to the kitchen to warn you, but she walks into the kitchen and sees you, and she has to come up with a reason for her to be there.
(I have some other theories too)
I think it was pretty fun to play, I love those interactive vídeos.
l
))))