Ah, my bad. I didn't think to check that. So... He wouldn't have been under Carter's command. Under a marine officer's, but they'd have probably met already. And as far as Horace's adventures go...

Carter would ignore the knife, already knowing Horace's level of aggression. Though the disobeying orders to attack, he'd tell him that if he planned anything like that, to talk to him. If it was tactically sound and safe for the men, it could be conducted.