by NPCWhiteMage Tue Mar 12, 2013 12:47 pm
Its no secret that Seath's goal in life is to replicate the scales of immortality that he had been robbed of in birth. It was one of the reasons that Seath turned on the Dragons: jealousy.
So, Gwyn grants Seath Dukehood and gives him a great library so that the dragon may continue to find a way to duplicate the scales of immortality.
I believe all the crystals are Seath's expiriments for those scales, but aren't quiet right, which is why they are associated with the curse.
Now about Logan going mad.
His whole goal in Lordran is to seek greater knowledge and wisdom.
When he first came to Lordran he was with Griggs, who abandoned Logan the first chance he had "For his safety..."
You first encouter Grande Sombrero Logan in Sen's fortress, where you free him. As a reward, he teaches you spells. After you make it through to Anor Londo, it can be assumed that Logan also made it to Anor Londo and indeed to the Duke's Archives, only to find the way blocked by the Gold Fog, dishearted, Logan returns to Fire Link where he continues to teach you; however, once the Lord Vessel has been placed, Logan sets off again to the archives in seard of knowledge and wisdom.
You run into Logan once again, and he teaches you crystal spells. Now, these spells weren't available before he made it to the archives, so it can be inferred that he learned about them while in the archive. Now, it can be inferred that Logan encounted Seath while in the Archives, and became cursed, which affected his mind (I feel this is referenced by this Tin Crysstalization Catalyst and use of Crystal spells). The curse affected his mind horrible, and Logan began to identify with Seath, both of them great intellects and masters of scorcery. In order to closer emulate Seath, the dragon that gave Logan the "gift" of the power of crystal spells, Logan takes off his clothes so that he too is scaleless. Having obtained his goal of greater knowledge (while not so much wisdom) Logan hollowfies, and loses his mind, a process that has been slowly happened throughout his time in Lordran.
This is just my take on the subject.