‘Y'know, you presuppose that the killer is a man. Well, what if I manage to flush out that the killer is a woman?'

He doesn't understand why she's so fixated on Sam all of a sudden. He doesn't understand why Keats has found a way to lurk about like a bad smell, and he doesn't understand why she, with all her knowledge, can't see that the bloke's a bastard. All in all, there's quite a lot he doesn't understand, but nothing's going to get in the way of solving this case. This one's important.

No way in hell the killer's a woman, though. He's just saying that to try and deflect.