He could also become very withdrawn and bitter. Draco might start to question his own judgment in ever thinking that Harry was someone he could co - exist with. This could lead to him becoming more isolated within the Slytherin house and focusing only on his own self - preservation and the interests of his pure - blood family.
Draco would probably be extremely angry. He might feel a deep sense of hurt and betrayal, especially if he considered Harry a friend or at least an acquaintance he could trust. He could start plotting revenge against Harry, using his own cunning and Slytherin skills to make Harry's life difficult at Hogwarts.
He could also react with suspicion. Draco might think that Harry has some ulterior motive, especially if the confession is of a more positive nature like friendship or love. He would question Harry's sincerity and might even test him in some way.
Another reaction could be confusion. Draco might not know how to feel. On one hand, he has his Slytherin pride and his clothes are part of that. But on the other hand, seeing Hermione in his clothes gives him a strange feeling in his stomach. He might end up being more curious about Hermione and her reasons for wearing his clothes.
Snape might also keep what he saw to himself at first. Instead of immediately confronting them, he could choose to observe them further. He might be curious about what they were really up to and use his skills as a spy to find out more. This way, he could gather more information before deciding on the best course of action, whether it be punishment or something else entirely.
At first, Draco might be in shock. He didn't expect Harry to break up with him at such a crucial time. Then he could try to reason with Harry, begging him to stay for the sake of their unborn child. When Harry refuses, Draco could turn to his own family for support and close himself off from the outside world for a while.
Harry might be very startled at first. He could jump up and reach for his wand, thinking it's some sort of trap. But once he realizes it's Draco, he would be confused. He might demand an explanation right away.
Draco would likely be disdainful at first. He has a high opinion of himself and might not like how some fanfictions depict him. Harry might be more curious and open to it, as he's always been more accepting of different ideas.
The wizarding world would be in shock. Harry is seen as a hero, and for him to kill Draco, even if Draco was from a 'less popular' side, would be a huge scandal. His friends like Hermione and Ron would be deeply conflicted. They would be torn between their loyalty to Harry and their disbelief at his actions.
Draco might become withdrawn. He could isolate himself from his friends and spend more time alone in the Slytherin common room, brooding over what Harry has done to him.
Harry might be initially shocked. He's used to seeing Draco as this tough, arrogant Slytherin. But then, he could feel a bit of pity. He might approach Draco tentatively and ask what's wrong. Harry has a good heart, and despite their past differences, he wouldn't just leave Draco crying. He may even offer some words of comfort or a handkerchief if he has one.
Harry might be initially angry. After all, he's been hurt. But he could also be a bit confused, especially if he thought Draco didn't mean to do it. In some cases, he might just walk away to cool off and think about what happened.