【Parallel Fates: Uchiha Itachi and Hozuki Mangetsu】
I see many parallels between Uchiha Itachi and Hozuki Mangetsu. Given their similarities in the original series, it is highly probable that Mangetsu’s cause of death was the same as Itachi’s: physical collapse due to pushing his body to the absolute limit to protect the village where he was born and raised.
【I’ve imagined a specific scenario for how this might have played out】
Suppose Kirigakure was in the midst of a war, facing an invasion from another village, just as Mangetsu’s body was reaching its breaking point. Imagine if Orochimaru—who coveted the Hozuki clan as experimental subjects—appeared before him and offered a deal:
The Cursed Seal: Orochimaru would grant Mangetsu a Cursed Seal, allowing him to gain the strength necessary to surpass his physical limits and fight on.
Protection for Suigetsu: Even if Kirigakure were to lose the war, Orochimaru would take Suigetsu under his wing and guarantee his survival (albeit as a test subject).
In this scenario, Mangetsu would accept the deal, eventually losing his life to the Cursed Seal for the sake of his village and his younger brother—just as Itachi sacrificed himself for Konoha and Sasuke.
【The Parallel Between the Younger Brothers】
If this were the case, it creates a striking connection between Sasuke and Suigetsu’s backstories:
Sasuke lost his brother due to the circumstances of Konoha. However, following his brother’s death, he was ultimately looked after by the village and eventually dedicated his adult life to serving it.
Suigetsu, in this theory, loses his brother due to Orochimaru’s intervention. Yet, his safety is secured as Orochimaru’s subject, and he continues to work under him into adulthood.
In both cases, the elder brother's death establishes both the safety of the younger brother during his youth and his eventual "career" as an adult.