His definitely saying "let her die " you're right and it's on purpose! and I can't believe how misunderstood this song is ! This song is not about unfaithfulness, it's about a girl who's DEAD !! yes the brunette girl is dead ! and his imagining himself speaking to her on the phone.
The evidence is in the video clip :
1-Notice during the clip we never see the brunette girl holding the phone talking to him back, we only see her in black and white footage symbolizing his old memories with her. This is not a coincidence, it has been purposefully teased to indicate that the girl who he truly loves his dead. and If that doesn't convince you..
2- The scene where the entire band shows up, take a look at the place they're at, more importantly notice the candles ! we see a lot of lighten candles up numerous times in the official music video ! again this is on purpose to symbolize that sadness and grief for the dead or maybe a they're playing the song at her funeral.
3- notice that when the camera switches to his back, when it's suppose to represent the point of view of his new girlfriend, the phone does not appear in his hands ! that's because the blond girlfriend does not see the phone ! she's living in reality while the singer is living in fantasy , whilst imagining the entire phone conversation with his old dead girlfriend, who he feels guilty for "cheating on" even whens she's dead.
4- THE LYRICS : notice " my girl is in the next room, I WISH IT WAS YOU" , "LET HER DIE" yes his saying it, it's not "and I", whoever is saying that it's "and I" is probably to first chorus when he should be listening to the lost chorus in the song ! . Listen carefully it's so obvious : "let her die, don't want to say goodbye" his contemplating the idea that the new girlfriend dies instead of the old girlfriend ! ( so the old one could come back)
5- The song's name, lips of an angel ? .... angel ? ANGEL ???? she's an angel because she's in heaven meaning she's dead !
6- The new girlfriend is expressionless, she's looking at him from the back and she looks confused ! why do you think that is ? because she's looking at him while he talks to himself ! she thinks his crazy. Again the lyrics "NO I don't think she has a clue !" his actually referring to how confused the blond chick is, because she doesn't know that his imagining a fictional conversation between him and his old dead girlfriend.
The evidence is there and this is not coincidence. YOU WELCOME