Or something close. The bomber's handlers needed ways to control him. Killing someone's best friend is like having him wake up next to his horse's head. Also the handlers could keep the case hanging open as blackmail.
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.