I've had that theory about Thirteen since s11 but I was surprised at how much this two-parter validated that a) she really does keep her team at arm's length b) she really does act differently when she's not around them, more importantly, she acts very differently when she's around someone who knows her really well.

She's a lot more still when she's around the Master, less manic, and a whole lot of steely.

And the Master admitting point-blank that he goes around killing to gain her attention, which is... I'm such a Doctor/Master shipper whichever the incarnation it seems.

But also, whatever the Master discovered did seem to have broken him, it's like he's clinging to being murderous because that's the only identity he knows.