If both salvage yards are owned by JK, then it seems likely to me that his properties were targeted for burglary. Isn't it possible he caught these 4 men in the act and they attempted to flee, driving their bikes into an adjacent area, where JK shot them in pursuit? This would complicate matters for JK if he called authorities because his actions would not be considered justifiable. In such an instance, JK's life would not have been threatened because the men were fleeing and hadn't harmed him physically. And while it might have been true that the men had stolen from him, and JK acted out of instinct and anger in shooting them, JK knew he could be convicted for his actions in court.
Perhaps, with adrenaline running, he made the quick decision to get rid of men's bodies by dismembering them and putting them in the water. Maybe he was hoping that the authorities would automatically assume someone linked with the cartel had done this.