I have said this before and no doubt will do so again.
In these type of crimes the death penalty would actually encourage more people to do the same!!!
These people genuinely think that by 'dying in the name of Islam' they become martyrs. They are told and believe that a Martyr, is on death, instantaneously transported to Paradise . In Paradise they will enjoy perfect health with no suffering or pain,wonderful food, and everything they ever need including an endless supply of willing, beautiful, seductive virgins. (including boys if that is their preference) Further they will be worshiped as heros for all eternity!
With that on offer getting life sentence is a disaster . They had intended being shot dead by the police so as to become Martyrs, the outcome is the worst that they could imagine and to them infinitely worse than a death sentence .Forced to live here on Earth with infidels!