And imo, it has to be that. Since Fuuma held the cherry blossum(that came from Hokuto) and pointed at Subar and Seishiro and said "but I think these two are happy". Most would say that Sei was killed by Subaru but in reality, he was killed by Hokuto. Sei knew what kind of spell Hoku put on him, and yet he tried to kill Subaru so he could get killed.