similar to counter strike, when you kill only one or two hostages you just lose cash, but if you kill too many of them you get kicked
in payday's case it could be:

• insta kick
• insta ban
• insta custody
• lose more cash
• become a priority target for the cops (this is actually already another mod's feature but could be implemented in this request too)

"why?" 'cuz I'm tired of genociders killing everyone on sight and not having enough body bags to handle with the corpses in heists such as go bank where infinite civs spawns