I find that if you look around, Seath has the largest tangible impact on the game, some of which is speculation, but is pretty valid.
For example, many of the enemies you come across are quite obviously (and some less so) his creations, such as the Crystal Golems, Moonlight Butterfly's, Crystal Hollows, and of course Seath's eyes and ears, the Channellers. Even the Crystal Lizards are believed to be his.
He's at least partially responsible for Havel being locked in that tower. Think about it. Havel hated magic and dragons, and thus would not have liked Seath at all. This would have lead to a shared animosity, considering how well the dragon reacted in the past to being shunned. When Havel was locked in the tower 'for his own good' as one item description says, I believe Seath was responsible for letting him out again when Gwyn returned, and seeing as he never did, Seath never bothered. I like to think that's why the key is found on a Moonlight Butterfly, one of Seath's creations, so that Havel could never escape.
It's pretty much assured, even if never explicitly stated, that Seath is Priscilla's father, though the mother is unknown. However, considering his obsession with kidnapping maidens and performing horrifying experiments on them, perhaps there is no mother, and she is his own attempt at splicing a daughter via magic?
There is a Channeller waiting in the church where Rhea goes to, and shortly after she disappears and ends up in the Archives, hollow. This is obviously his doing, direct or not. It's also possible the Channeller is why Lautrec is locked away.
He's obviously completely mad, considering what he's done to himself. Being born albino with no scales, no legs, thin insect wings, possibly no eyesight and no immortality drove him to research ways of obtaining said immortality. His jealously over his fellow dragons having their scales is what drove him to betray them to Gwyn and is implied to be the a big factor in how they won. His obsession with immortality moved him to the crystals, and this could have lead to several things happening, including the crystalification of many creatures and things, such as perhaps the Crystal Knight, and all the Crystal weaponry that can be found. Crystallising himself partially may have been what he did before he learned of the Primordial Crystal.
And of course, we can't forget that he was the grandfather of sorcery, and thus has an indirect hand in every spell you or anyone else casts. Which brings me to Logan, who goes just as mad as Seath does, and even sheds his clothing to try and imitate the white beast. Seath's obsession with crystals as well as his affinity for magic may be where curses come from.
I like to think, with his penchant for kidnapping maidens and experimenting on them, his creation of countless horrors, including his 'new and improved self', and his apparent chronic backstabbing disorder, Seath is one of, if not the worst character in the game. He is a complete monster who chose to do most of what he did before he went mad, and even after that it continued, horror after horror. It brings me a cold satisfaction for me to kill such a sick creature.