Interesting. What people in the comics seem to be reading is Numair repressing his feelings in an unhealthy fashion. What I read was Numair fighting to get his fury under control because he understands that erupting in rage at the moment is only going to make things worse, and because he thinks that any hope of straightening things out with Hawk isn't going to be reached by screaming at him (or, worse, attacking him in some fashion; Numair's not the kind of fighter that Hawk is, but he does have some effective access to magic.)
A ruler has to be able to control their anger, if they're not to become the sort of tyrant who murders others for disagreeing. Control it, not ignore it; anger's a very useful thing, but only if it's under control. Maybe the commenters are correct and Numair's trying to ignore his anger instead of control it for later use when and as appropriate; but I hope not.