Originally Posted by Son of mortarion
It does seem as if the reasoning is iffy, more an excuse to have grey knights that are servants of chaos because you want grey knights, than a real, "it could happen in the 40k universe" reason.
Instead of trying to make a grey knight that serves chaos, how about making a servant of chaos that is equivalent, like Red Orc said. Instead of a nemisis force halberd, a minor daemon weapon, instead of psycannons, warp infused heavy bolter/autocannon/stubber type weapon, there is no reason that you can't take the rules, apply the "counts as" principle, and then create a good, fluffy unit. Just avoid any reasoning that requires a '"what if," that tends towards rationalizing bending/breaking fluff and or rules for who can ally with who.
But then their not grey knights. If you combined normtheunsavory's idea with that it would make more sense, like the first grey knights were faulty so they decided to kill them but the faulty grey knights found out before they were killed so they ran away and gave their fealty to chaos because they felt they had been betrayed.I would also look cool, with a deamon halberd and stuff like that