Bob, is a Cleric, who was a member of Father Octavian's mission to track down the last Weeping Angel which was on the Byzantium when it crashed. During the expedition, in the "Maze of the Dead", Bob was killed by a Weeping Angel when it snapped his neck. However, The Eleventh Doctor, could still communicate with Bob via Father Octavian's communication device. Because the Angels lacked a voice and because it wanted to torment its victims it copied Bob's consciousness and used him has a mouthpiece to talk to the Doctor. While it sounded calm the words clearly showed the psychopathy of the angels, and how they didn't care about killing innocent men. The angel simply stated it needed a mouth piece so it just took one for itself.
This Angels lives up to the Doctor's claims about them being the kindest of killers because the Bob's conciounesss mentioned that the Weeping Angel snapped his neck which "was quick and didn't hurt as much has he thought it would."
Bob's conciousness confirmed the Doctor's suspision that the Byzantian Crash was a rescue mission on the Angel's part and while its bretherin were still feeding on the ship's temporal core to heal, and that it was coming for the Doctor and the others and it would enjoy killing them.