It's standard in criminal organizations that, in order to be admitted into the inner circle you have to commit murder (or some other serious crime). It weeds out undercover cops. It means that you cannot safely testify against any mob member without your own crimes being exposed.
I lean towards this being an initiation rite. Particularly since at least two of the victims were Jewish.
The reason for my speculation (pure speculation) is that the bomber was already long in the inner circle (Islam). He had no other higher calling than to the Jihad thing, not join some gang to be cool, even a Jihad gang. OTOH the Saudi and Chechen moneymen had good reason to want him (a mostly white guy).