They’ve never spared enemies in the middle of a battle. They’ve only spared them at the end of battles, and the series has how having actual messengers and/or P.O.W.s can have benefits even in Vernichtungskrieg type wars.
Only King (and eldest brother) are true nazis though, the others are just following their commands,
Taiga sparing 2nd brother Georg did have some effect on him, even if he was too weak willed to go against his elders.
so peaceful resolution is still possible since they now know their eldest brother is a douche who can't be trusted.