I think it was Scarlet
First, Scarlet overheard White and Wire talking to each other about the electricity. After they were done, Scarlet entered the room, wearing a mask. There, she planted a trap, and also took the "how to make a table" book with her as she was leaving the room. At lunchtime, White walked to her room to get the "how to make a table" book, which Scarlet had already taken at that time. White throwed some books of the bookshelves to find the book. That's when Scarlet comes into the room, and punched in Whites gut (she might have used the book for that, but i don't know). Then, she made cuts in both of White's arms with the knife she got somewhere, which made him bleed out and die. Then, Scarlet destroyed the electicity, which made the lights go out so nobody could see Scarlet suddenly enter room everyone was in and make her the only one without an alibi. After that, she also used the knife to cut the rope of the trapdoor, which made White's body crash into the unfinished table (that's why he went to get the "how to make a table" book), which broke the table. The impact of the fall broke White's legs.
Also, i think it was Scarlet because she was one of the three to have masks while entering the mansion. It's possible that someone stole them, but i can't confirm that.