ONLY time I'll EVER invoke slippery slope. If we make an exception for mass murderers, what's stopping us from hanging people who committed other crimes and should have gotten life without parole.
The only exception to my strong opposition to the death penalty is that I'm glad Bin Laden was killed.
I don't think there's a slippery slope for use of the DP if convicted of multiple murders committed on separate days.
I read the question as a single murder by a person with a previous record of sexual assault, or with that sexual assault as part of the murder. I would not generally favor the death penalty in that case. If however it was for a subsequent murder I think it should be eligible for the death penalty. This leaves DP as a punishment for serial killers. It also provides for a harsher punishment for murderers who kill again while in prison for life.