There is just a super subtle grinding of his jaw if you watch closely. Like he just wants to kill everyone, but wants to get his crew more so he controls himself
Khan is potentially dangerous and all, but he’s not a mindless animal though… His mind is brilliant.
This is right as Marcus is trying to convince Kirk that Khan and his people are, basically, too much of a potential danger to be allowed to live. A position that mankind in general tends to take when it comes to augments being allowed to live or not (hence the Botany Bay’s self-imposed exile from Earth, and other examples such as Cold Station 12’s thing with the babies).
So it’s very likely that this look of hopelessness and the little movements his face is making are caused by the mixture of despair and worry that even Kirk, the one who has been the exception to this rule so far, will also come to see things the way Marcus does, (whether by true conviction or to avoid endangering himself and his own by protecting Khan any longer). Thus destroying the very last line of defense Khan and his crew have.
This is going on live as he’s listening to the broadcast, waiting to know if his family and him are about to get another postponing to their potential imminent execution, or if it’s time to fight one last hopeless battle (and alone, not even Khan can truly win this.)
So I’m pretty sure that it’s not about mindless blood-thirst, it’s about the nerve-wracking wait to know if you have another chance to fight for your life and that of your loved ones, or if the very last person still willing to protect you is about to change sides and try to feed you to the wolves…