Marcello stood against his father because he understood the consequences of his father's reckless actions. It wasn't just their protection at risk—their infighting would invite other families, enemies who despised theirs, to exploit the weakness.
If things continued like this, the family would collapse. Or worse, its very core would be lost.
There would be no recovering from that and this was the last thing Marcello wanted.
He understood his father's approach to things were archaic and they needed change but Giovanni's way would turn them into nothing more than a violent gang.
Marcello knew he couldn't let that happen. He decided to confront the head of the opposing faction directly. He had to make them see reason otherwise they might as well destroy everything they had built.