The "logical" reason was that Zasa intended to have all of them whacked, whether or not he got a check. Then he'd be undisputed boss.
The real reason was to provide FFC with an opportunity to put on a showy, hi-tech massacre--to compete with the showy, hi-tech massacres in the other films of that era.

Ntra la porta tua lu sangu � sparsu,
E nun me mporta si ce muoru accisu...
E s'iddu muoru e vaju mparadisu
Si nun ce truovo a ttia, mancu ce trasu.