As a Maester in the temple of Yevon, Seymour was adored by the masses and super-powerful to boot, boasting arguably one of the coolest summons in FF history, Anima.
To put Seymour’s good-guy-gone-bad twist into context, imagine if the Dalai Lama were to all of a sudden decide he was the man singlehandedly tasked with ridding the world of evil, and in order to do so, he had to become evil.
I mean sure, I can see where you’re coming from, but it's also not cool - or is it?!
Anyway, Seymour’s exile from his own race as a child, coupled with the death of his mother and a sense of crippling solitude eventually convinced him that becoming Sin was the best way forward and a way to finally release all of Spira from its cycle of dread.
Think about it, every now and then, this monstrous entity rears its ugly head and plunges the world into peril, with the races of Spira just praying that generation’s summoner champion is strong enough to defeat it. Imagine a world where Donald Trump is re-elected every three terms - it just wouldn’t work.
Maybe Seymour had it all figured out.