Dying To Be Martyrs
As best I can tell, there was no tactical necessity for the London bombers to die.
No, the four men that died voluntarily in the blasts did so simply because they aspired to be egg timers. It was, apparently, the only role in the attack that they could imagine for themselves in which they got to die. And a glorious death was the goal around which everything else was planned.
Mac Johnson looks at the culture that creates homicide bombers and has some pretty good ideas on how to combat that culture.