I thought that was long-established canon, and the rituals they go through to become GK is what keeps them from being corruptible..? Or something outside one guy's mind that made them that way?
The rituals make them very hard to corrupt, but the canon before the Grey Knights Codex was that, while no Knight has ever fallen to Chaos, it is still entirely possible. Many of them have gotten close to the brink and to the point where Marines of other Chapters would have been killed for heresy. They have more Inquisitorial leeway than other Marines, but it is absurd to say that it is impossible or them to fall to Chaos.
This is the same chapter that chased down IG regiments that fought Abaddon to kill them so word of Chaos would not get out...
Yes, but the Sisters of Battle are another part of the Inquisition. Not to mention that killing them and bathing in their blood is something Chaos would do. Hell, even if I take Ward's bullshit excuse, it conflicts with his declaration that Grey Knights can't be corrupted anyway.
UGH! I can't stand Ultra-Smurfs... They're so perfect they are annoying...
My big problem with them is, with a few exceptions like Captain Titus, they're BORING. There's nothing that really makes them stand out. The Dark Angels have their secrecy and the Fallen, Blood Angels have stuff like the Death Company, the Space Wolves have.... every fucking thing about them, but the Ultramarines are just Marines in blue armor.