I totally understand Seymour and I can't say I disagree with it. His way is just a different means to the same end Auron hoped for. A way to break the cycle of suffering on Spira, but unlike Yuna he had nothing worth living for. His understanding of the world is skewed by his religion and by his loneliness.

On Spira, if people die their consciousnesses remain, they even appear as they did in life, and can enjoy things similarly. So by killing all of them, by becoming the last Sin, the cycle ends and everyone would be apart of him and apart of each other in a weird and mystical way. Spira's people could continue without change, Yevon gets what they want and, the world no longer suffers from the constant death caused by sin.