Maybe it's a combined conspiracy by everyone? 😲 😆 (Those who've read the excellent mystery, And Then There Were None, probably understand what I'm trying to say better than others...😉😳)
The guy that we couldn't see...his voice did sound familiar, but maybe it's just our conscious, because we know it's someone from the family.
I'm thinking, that's the murderer, but he was hired to do the killings by someone from the family. He was talking to a lady! So that really narrows down the possibilities...