He may be filled with remorse. Remembering all the times he was harsh on Harry, he might start to cry, tears falling onto Harry's face. His voice could be choked with emotion as he whispers apologies or words of respect for Harry's bravery.
Snape could be in shock. He might freeze for a moment, unable to believe what is happening. His face would show a mixture of horror and disbelief as he holds Harry's dying body.
Snape could initially be skeptical. Given their long - standing enmity, he might not believe that Harry's apology is sincere at first. He might respond with a cold glare or a sarcastic comment, like 'Potter, your apologies are as empty as your head.'
However, as the story progresses, Harry may start to understand the reasons behind Snape's actions. He could become more resigned and accept the punishment as a part of his growth. He might use the experience to become more cautious in his actions and more respectful of the rules.
Snape might initially be surprised. He could then use this knowledge in a rather Snape - like way, perhaps to gain an advantage in his complex relationship with Harry. Maybe he'd be a bit more guarded in his interactions with Harry, or he could even try to subtly help Harry overcome his fear while still maintaining his tough exterior.
He would probably be in shock. Snape has spent so much time around Harry in one way or another throughout his years at Hogwarts. His immediate reaction could be disbelief. He might stand there frozen for a moment, unable to process what he's seeing. Then, the full weight of the situation would hit him, and he could break down in a rare display of raw emotion.
He might be extremely shocked. Snape has his own views on the Malfoy family and Harry Potter. Given his complex relationship with both, he could initially be in disbelief and then perhaps a bit angry that Harry would be a fan of a Malfoy.
In some fanfictions, Snape might awkwardly try to comfort Harry. Given his own complex personality, it wouldn't be an easy or natural gesture for him. He might pat Harry's shoulder stiffly or offer some gruff words of reassurance like 'Pull yourself together, Potter.'
He could be secretly worried. Snape might use his knowledge of potions and dark magic to try and find a cure or at least a way to stop the bleeding. He may also be conflicted, as his past relationship with Harry and his loyalty to different sides in the wizarding world play a role in how much he wants to help Harry.
Snape might be initially shocked. He has his own preconceived notions about Harry, and this revelation could completely change his view. He might struggle to accept it at first, given his traditional and somewhat rigid personality.
Perhaps in the fanfiction, there's a curse that was meant for Snape but Harry jumps in to take it. As Harry is dying in Snape's arms, Snape has a moment of deep reflection. He might think about all the times he could have been kinder to Harry, and how Harry has now become a symbol of the good that Snape had long forgotten in his own life. And this moment of Harry's death could be a turning point for Snape's character development in the fanfiction.
At first, Harry is likely to be angry. He might feel that Snape is being unfair and targeting him. Harry has a strong sense of justice, and when he believes he's being wrongly punished, he'll be resentful.