because the suffering and etc is not eternal like he believes it is.
there is an amazing spira with hypello and what not out there and he thinks in his crazy mind we should kill them all because???
he doesn't really care about sin other than as a tool of power for himself personally.
he wants to consume all of spira into a kingdom of death, and he basically just wants to see the chaos unfold.
it's his unknowing ablilties in the game that make him a worried protagonist
because he is really strong in game
and he also is a grand maestor etc.
ffx was sin and the story of tidus and yuna coming to deal with that as many pilgramge but this time, realities converged etc, and as tidus and yuna were solving this issue like mature adults
seymour saw an oopurtunity to lunge in.
yuna was ataken by his negotiating power to his populus support which yuna really
cares about people as natural
and yuna saw what he was doing she didn't communicate with the team sadly, but she saw how we would need to deal with this at some point in future, so forced it to happen
which really comes to the new story of ffx3 where--- anyways I love ffx lore we are all gonna see this magic come shortly.
so she intersected us into seymour and we had to go and hunt him down basically he just turned into a crazed cultist but he had amazing music