It occurs to me as I reflect on Seymour's weaknesses as a character that FF only knows how to write one way of a villain acting, despite their myriad personal differences and varying levels of sympathy. It first struck me with Seymour because, while he is supposed to be insane, he's also supposed to be well-intentioned in a warped way. I like to contrast him to Dagoth Ur from Morrowind. When I played Morrowind, me and other players discussed his charisma but somebody else said it wasn't charisma so much as "it's refreshing to not have a villain rant about having to break a few eggs to make an omelette." Dagoth Ur is crazy but he also seems genuinely kind and benevolent in his own twisted way. He is genuinely doing what he thinks is good for everyone. Seymour talks a lot about easing Spira's suffering, and we're clearly supposed to believe that is his goal, but all he does is laugh about murdering folks and that rather undermines his messianic delusions.

And that sort of MWAHAHAHAHA evil is present in every FF main villain with the exception of Vayne. From Sephiroth on they are pretty much all supposed to be tragic and pitiable but every single one of them, from Sephiroth to Ardyn, indulges in sadistic cackling. It's kind of hard to believe there is anything but pure malevolence here when they keep relishing in the violence they perpetrate upon others. Lotta people don't wanna forgive Kuja and it probably be much easier to see his perfectly human struggles if he didn't keep laughing at everyone's misery.

Wait, I was forgetting somebody, Caius in XIII-2. He's angry and full of spite but never once has a long evil laugh about hurting people while also claiming he's helping. Caius continues to be the best FF villain.