Once we have the timeline straight between the werewolf incident and Snape’s Worst Memory, there’s one thing that jumps off the page: the creation of Sectumsempra.
And boy, is it a nasty spell.
Unlike the other creations littered through the Prince’s Potions book, this one is clearly labelled: For enemies.
Sectumsempra is akin to an invisible sword. Wielded correctly, it’s a hex that can slice, dice and ultimately, kill. Lupin commented years later that it was Snape’s specialty, so he clearly practiced and honed his skill.
Sectumsempra is a huge departure from the other created spells found in the Prince’s Potions book, which are either useful (Langlock; Muffliato) or arguably jocular in intent (toe-nail growing hex; Levicorpus). In contrast, there’s no jokey element to Sectumsempra; the intent is to cause great harm.
Langlock is the first spell that really intrigues me; it has a very deliberate outcome - and why would you want to stop someone from talking? Well, in your early years at Hogwarts, it’s how you prevent someone from spell casting, because younger students aren’t skilled in non-verbal casting.
So I think that points to Snape trying to defend himself when being attacked four-on-one; but it’s curious that none of his early spells are particularly nasty or vicious, and that Sectumsempra is such an elevation in horror.
So what changes?
Enter: the werewolf incident.
I think this broke Snape.
I think he’d handled the four-on-one bullying for as long as he could, and Sirius had unwittingly changed the rules of the game. It wasn’t “just” bullying; it wasn’t just harassing and irritating and picking on him. It wasn’t just calling him names, and flicking some hexes down the corridor.
This time, they tried to kill him.
And when we look at the timeline, we can see that they didn’t have the decency to leave him alone after that…which implies that they can’t have been severely punished for their actions, because they show little remorse.
That in turn suggests that Snape probably felt isolated. We already know that Lily had sided with James Potter’s version of events, and if the Marauders weren’t punished harshly by the teachers, then he probably felt as if they wouldn’t defend him in the future.
So Snape created Sectumsempra. It’s the magical equivalent of a bullied kid taking a penknife to school to defend himself. He created a knife, and having learnt his lesson from Levicorpus, he unleashed it nonverbally, ensuring that nobody would be able to use it against him in retaliation.