Actually...
[spoiler=Pre-Split Chapters, then Conquest spoilers]Yes, his orders at the time were, in fact, to kill Corrin and Gunther by order of King Garon himself after sparking off the war - he says so outright. (The fact he does this kind of does set the table of 'Hoshido good guys, Nohr bad guys' a little too handily, I think...)
I don't recall exact details, but he appeared to largely try to stay 'professional' about it all in the wake of Corrin's return. When Corrin says 'no, stop' to the massacre he does in front of you later on in chapter 13, he uses his 'nervous' portrait as if to say 'uh, yeah, I can't, it's the king's orders.'
This is why it's so very interesting to me that he claims that Corrin relished in murdering the Cheve rebels in front of King Garon - it's inconsistent with much of his previous characterization. If he's all about saving face in fulfilling orders as to have what he really seeks out of doing so, that seemed almost a professional courtesy on his part rather than any sort of overt apology for previous behavior. He's a hired thug, he knows it, he relishes in it - but I think if Corrin looked bad it'd reflect back onto him too since they were part of the same operation there.
It still really could be just him saying it to annoy Corrin by the writers' intent (as he'd be in a position where he couldn't verbally nor physically retaliate for it), but that's just not how it came off to me. I'd like to think it was a grasp at character depth that doesn't get expanded upon.