The historical assertion is in the context of group political conflict involving violence.

Apocryphally, the Mafia had a rule that they didn't target families. Assuming this was true even in conflicts between families (and I don't know this to be actually true), I'm saying that the family (Blue) that first broke that norm was not likely to be the family that unilaterally returns to the "don't kill dependents" rule. Particularly if a lot of the unity of the Blue family is based on the strong personality of Blue's leader.

(Mostly, I'm thinking of the Cold-War era internal conflicts between proxies of the United States and the USSR, especially in Latin America and Africa i.e. the Sandinistas. I'm not saying every conflict of that type escalated in the way I'm describing, just that the escalator basically never deescalated while the conflict continued).