He is also thinking that this is incredibly hot, it turns out.
The rest of it is more complicated - firstly, it was bad behavior, you obviously shouldn't change the rules during really serious sparring, Leareth would be justified in being angry but also Matirin wouldn't have done it if it'd be bad for Leareth, he knows Leareth well enough to make decisions like that, it's impolite but it's safe, toeing the real boundary instead of the one people are confined to when they might be wrong. But it was bad behavior and he can feel faintly apologetic, on some level, while also - there's some kind of power, see, in knowing someone well enough to know when you can break the rules - (and he was, actually, confident in his own survival, he's in morph, he's seen Leareth fight for real)
(It's incredibly hot.)
- he's been building a picture, for the last ten rounds of this, of what it's doing for Leareth, he started out confused but he's been getting steadily less so and at this point he can understand it much better than he can put it into words, but Leareth needs - needs competition without superficiality, needs to win on his merits at things that really matter when the stakes are not imaginary, needs to leave this awful unfair world behind in favor of one that's much simpler - and sparring does that, does it really well, but there's a level on which the sparring isn't real, because Leareth is holding himself back from using almost all of his power. And Matirin wanted, very badly, to see what it was like when he wasn't.
(It's incredibly hot.)
And beneath all that he's sad, and hurt, and directionlessly angry, and tired in a way that isn't about having spent the last hour sparring - they both are - and this is bright and blazing and different than that and he needs it very badly and this line of thought is bleeding into the one about how attractive Leareth is, again, about how appealing this is which he wouldn't have expected in the slightest, it's the sense that it's real, it's not pretend, but he trusts Leareth totally, and loves him, and wants to fix the worlds with him - and that makes it more real, rather than less so -