Never try to understand Baatezu career paths (https://docs.google.com/open?id=0B-ECdSyQNz_2M2MxNmIyOTYtMzQ4Yy00MmQyLWFkZjUtNzg0ZTI3MTVlYzIx&hl=en_US). Just don't do it. I'm about 2/3 of the way through adding all of the different forms of Devils to that flow chart and my brain already hurts.