Board Thread:Warhammer 40k General Discussion/@comment-174.29.74.18-20130615110043/@comment-3580035-20130615121544

Inquisitors turn heretical all the time. There has only been a single case of a Sister of Battle willingly turning to Chaos. No Grey Knight has ever turned to Chaos.

You should read up on some of the fluff a bit better. Its even quite clearly emphasised that no Grey Knight has ever turned to Chaos, and the other info you can find simply by searching for it.