Why did Peter pour hot water over the Dr.?

I don't understand why Peter poured hot water over the doctor, how cruel! I thought he was to make it look like a heinous murder AFTER they were poisoned. He had cut the old lady's throat AFTER she died or did I misunderstand? Didn't he mutilate the bodies after they died to make it look like a serial killer? And did I sleep during the part where he "killed" the others. I only remember the 1st lady, the man in the barn, and the doctor. How did they figure out that the positions of the deceased mouths formed the word Libera? Just because they were killed from west to east? What was the logic in that? I didn't quite understand Peter's logic either about the history and the 12 apostles and their special prayers to resurrect a body. I guess I'll have to read the book to get it. But it was so mysterious to me. Hoping someone here would have already explained it. But if you know, let ME know! Thanks!