Spoiler maybe? My theory

I think the murderer is first victim, the one supposed to guide them inside the castle. He faked being dead to not raise suspicion, and was the only one with access to all phones, he was supposedly the only one with knowledge of the place, hence why the were all phones were collected beforehand and destroyed and put in the same hidden place. Also, if he knows secret passages he could attack without being detected. I think is very possible