I've been mulling it over and I really do suspect Kate. We've seen Steve get into a multitude of scrapes whereas Kate always gets away unscathed. I know she was working long hours with AC12, but she fell out with her husband because she was never home... busy being bent as well maybe? Also, if I’m remembering correctly, in Series 2, Richard Akers told Kate that before the ambush, Jayne Akers told him to get in touch with Kate (or something like that?) and Kate told him not to mention this when AC12 interview him. Did we ever get to the bottom of what that was all about?! Also, Dot taking a bullet for her could well have been so that she can carry on what he couldn’t...
I love the big three and agree it wouldn’t be the same without them, but at the same time I feel like we’ll only be satisfied if it is one of them that turns out to be bent/the top dog. Being in anti-corruption, they can expose just enough to look like they’re doing their job, but still prevent plenty of other stuff from coming out. Hmmm