Snape's reaction would likely be more complex than others. While others might show immediate relief, Snape might be more guarded. He has a history of animosity with Harry, so he might not show his true feelings right away. He could be more critical, even as he helps Harry, asking him how he got into such a mess in the first place.
Snape's reaction could be quite mixed. On one hand, he might be angry at Dumbledore for not doing more if he knew about the abuse. He may also feel a strange sort of empathy towards Harry. After all, Snape himself had a difficult childhood. So, he could start to be a bit more lenient with Harry in his classes and try to subtly offer support.
Well, Snape has always had a complex relationship with Harry. When he finds Harry cutting himself in this fanfiction, he might be angry at first, thinking it's a sign of weakness. But then, as he realizes the depth of Harry's pain, he could soften. He might start to see Harry more as a troubled child rather than just James Potter's son. Snape could use his knowledge of potions to create something to heal the physical wounds and also try to offer some form of emotional support, albeit in his own gruff way.
Perhaps Harry was investigating something related to Voldemort on his own and got in over his head. He could have been captured by Voldemort's followers who were hiding him away. It's also possible that a magical spell backfired on him while he was practicing magic in a secluded area and it made him disappear, only to be found later by Snape.
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.'
Snape would likely be angry at first. Harry has always been a source of trouble in his eyes, and running away would just add to that perception. He might mutter some sarcastic remarks under his breath about Potter's irresponsibility.
At first, Harry might be extremely suspicious. Snape has always been cold and mean to him in the past. He would wonder what Snape's true motives are. Harry is used to being on his own and not relying on Snape for help, so this sudden rescue would be very unexpected.
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 be suspicious at first. He might think it's some sort of trick or part of a bigger plot. However, as he observes Hermione more closely and realizes the genuineness of her situation, his attitude could gradually change to one of reluctant assistance. He might even offer some words of wisdom, albeit in his typical sarcastic way, to help Hermione regain her strength.
Snape might be secretly pleased at first, seeing as he has no love for Sirius. But then, being a teacher, he'd probably feel obliged to report it. He also might use this situation to his advantage against Sirius in the wizarding politics within Hogwarts.
He is often very suspicious. He might immediately think it's some kind of trick by the Marauders or their descendants. He'll likely try to figure out how to use it to his advantage against those he dislikes.