This moment in so much worse now that we know about Valka. because we can understand Stoick much deeply. He is SO angry with Hiccup for being friend with dragons! They took his beloved wife, so how can he accept them near his child? Also, why Hiccup doesn’t understand it? How can he give trust to the thing that killed Valka, his own mother? It is unacceptable. Stoick goes with “you have thrown your lot in with them” meaning “and not with us. Not with me. Not with your family!”. In that moment he is so furious he can’t forgive Hiccup for his behavior with the killers of his mother, and he hurts him, looking at him like he is one of them now, killing his wife again by being friend with such a demon. He can’t relate to Hiccup less, in this moment, and he denies Hiccup to be even his son. The moment he took his firsts steps outside the room, he is overwhelmed with consciousness. He’s swept away so violently that he nearly fainted on the spot: how could he say such a thing? We can clearly see his eyes get lost, being confused by thoughts and memories, and he’s surely thinking about Valka, about her same behavior, her diversity and how much she and Hic have in common… Then, here it is, he stops his “falling” and his expression changes again. His eyes are wide open, his mouth contracted, and he sighs: Valka died. Valka died for having this same attitude. Valka died for being different, for being peaceful with the dragons. That is, in the first place, why Stoick always tried to raise Hiccup in the way other Vikings are, trying to erase his dissimilarity. He will NOT let him die or risk his life for such foolishness… He NEED to protect him by destroying the nest even if this will hurt his son, maybe not more than Stoick did himself. And even after we see him close his eyes to blast away these thoughts, focusing on his mission, he opens them again… But the only thing that faded in his look is the fear of what just happened or could. The sadness and the regret are still there completely, because in one way or another, he will probably lose his son in his attempt to keep him safe.