So that smile, the one everyone always points to as his gentlest and sincerest smile in the whole show? Is also distinctly sad. Because he knows it’s over. He knows the final blow is coming. But when Mokuba comes running over begging for forgiveness, he forces a smile anyway. It’s like he’s going “no kiddo, shh, it’s not your fault, it’s fine, it’s gonna be fine”
And meanwhile he can see that blast coming from over Mokuba’s shoulder, you can see it in his face in those last couple shots. But it’s too late to do anything about it. It’s too late to save either of them. So in his final moments all he can do is put on a brave face for his little brother and lift his arm like he still believes they’ll reach each other in time.