Suo "Not caring" rant, essentially
So far based on chapter 216 and Suo leaving Furin, I think that Suo either left Furin since he wanted a place to escape from the RC (Red Chanpuru) and the responsability of leadership or he was sent on a mission to gain more allies/influence.
Suo as we now know, is a General in the RC and so making friends with Sakura, the first years, and making Sakura leader of their year/class, it would be like planting a seed in which Suo may benefit from. Strengthening Suo's relations with Furin where he could possibly manipulate the leadership once Sakura gains power after Umemiya graduates. Him not wanting to be leader also makes sense since Suo already is a leader and is capable without having straight forward control.
Sure, Furin is a group filled with teens and young adults. While RC is a faction that I assume deals with the authorities. But thing lik corruption, incompetent law enforcement and sheer human numbers come into play.
Furin is big group/school body with plenty capable fighters. Suo could have potentially been a spy to gain more allies, especially since RC is a multi-cultural faction.
Either was I still have faith that Suo deep down loves Furin, especially Nirei and Sakura.
Though its best to wait and see. (Suo's my favourite character so I gotta hold on lol).