At this point in the series, Kate is carefully observing Dot, and she's standing right next to him when he reacts to Steve's comments about Tommy Hunter's treatment of girls.
She's intelligent, she's playing everyone else in the room, and she's the only one who sees Dot's escalating panic. He's realising that he's losing control, and that he'll never escape the life Tommy forced him into. He's realising all this while Kate is watching.
The conclusion she comes away with is that girls weren't the only kids being groomed by Tommy. She says as much to Dot, right before he gives his life to save her. Her experiences with Ryan Pilkington as a child, and her superior investigatory skills, allow her to break past Dot's defences in a way nobody else manages to. She sees what happened to Dot.
If anyone could've saved Dot, it would've been her. And if anyone could've made him confess (bullets and blood loss permitting), it would've been her. Steve was too entwined with the case, focussed on justice for the children at Sands View. Kate saw what was happening and positioned herself as an undercover officer, because that's what she does best.
All this occurred while Hastings, in a matter of days, became convinced that Steve was a criminal. Kate was doing the hard work, pretending to distrust her closest friend, and identifying an enemy in their ranks-- while Hastings couldn't even stick by his (supposedly valued) employee long enough to see that he was being manipulated.
Which is all very wild to think about, when you consider the fact that Hastings refused to give Kate a promotion until Roz called him a misogynist... And he promoted Kate, predominantly, to prove that he wasn't.
Hastings doesn't appreciate Kate's hard work. He doesn't value her, and he never did. No wonder she left. No wonder she went to work for Jo instead.
I wonder how the series would've progressed if Hastings had been more competent?
