Have you downloaded the Amalathian sourcebook from Dark Magenta? It could help to flesh outbuis background. One thing I will say is that an ex-death-cultist is unlikely, and that a drill abbot seems a bit shouty-shouty for a very secretive Ordo.
Someone who survived an assassination attempt would be a good idea, though you'll need to be more specific- he's unlikely to survive an Assassinorum attack ++Cross Ref. Eversor=Destroyer++ but maybe he survived, and then decided to ensure that assassination attempts against other targets would not fail, and now tries to make assassins as good as possible by restructuring their training infrastructure. It's more of a Recongregator mindset, but hey, it sounds fine.
Keep plugging away- maybe just write a huge list of stuff (characteristics, looks, personality traits, potential backgrounds) and cross off stuff you don't like until you're left with the bare bones of a character who you like, and will be willing to flesh out more.