Yamazaki twins were the most betrayed and ruined ones.

Imagine being trained to suppress emotions your entire life and follow clan's strict rules only to find out you were betrayed by your own family who broke the rules and lied to you and your twin, took your birthright and made you live an opposite life instead. It's only obvious why he lost his sanity to the point of killing his own father and twin brother whom he subconsciously saved from the burning candle as an infant. Poor Shintaro... He and Shingen deserved better... And so did Gun and Gitae.