Terminus Prime was a planet where people could go to, to die at a method of their choosing.
Later he took Lily captive and killed her in front of River Song, realising that she was a proto-Time Lord but not knowing where she came from or who she was. However, Lily didn’t regenerate, as she had used up all her allotted regenerations. Furious, River shot Lake dead, only to be shocked when he regenerated into Lily. Realising what had to be done, River sent the newly regenerated Lake back in time so that River and Kevin could encounter her in the cave prior to her earlier incarnation killing her for the cycle to be complete. River then buried Lily's body. (AUDIO: The Lady in the Lake)