Perhaps an old enemy from Snape's past, someone who has a long - standing grudge against him. This person could have tracked him down to the Shrieking Shack and finally got their revenge. It could be someone related to his days as a Death Eater or even from his school days when he might have made some enemies.
Well, perhaps his death in the Shrieking Shack could be part of a complex revenge plot. There could be an enemy who lures him there and manages to overpower him. This could be related to his past actions or allegiances. For example, if some Death Eaters still hold a grudge against him for his double - agent role, they might target him there.
There's also the possibility that it was an accident. Fanfiction often plays with the idea of magical accidents. Maybe a spell misfired during a demonstration or a fight that broke out among the Death Eaters that Snape got caught in the middle of, and as a result, he got injured.
It could be the students. They might think Snape is too gloomy and old - fashioned in his appearance. So, as a bit of mischief or maybe out of kindness, they use their magic skills to give him a makeover. For example, Harry Potter and his friends could be behind it, using spells they learned in their Defense Against the Dark Arts class.
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.
Well, some characters' reactions can be really complex. For example, Dumbledore's portrait might show a solemn look, as if he knew this was an inevitable outcome. The students at Hogwarts would be in a state of shock. Those who were in Slytherin house might feel a sense of loss for their head of house, even if they didn't always understand him. And Neville, he might see Snape's death as a tragic end to a misunderstood hero.
One common theme is redemption. Snape's death is often seen as the ultimate act of self - sacrifice that redeems all his past wrongdoings. Another theme is unrequited love, especially his love for Lily Potter which is a strong undercurrent in many such fanfictions.
In some darker fanfictions, it could be a member of the Order who has always suspected Snape of being a double agent. This person might have decided to take matters into their own hands and attack Snape during the meeting, believing they are protecting the Order from a potential threat.
In the movie, Voldemort was worried about the rebound of the spell (because he believed that Snape was the master of the Elder Wand and that the Elder Wand would not harm its master, and he had experienced the rebound of the spell before). Instead of using the spell, he used the Elder Wand as a dagger to cut Snape's neck, and then called Nagini to bite Snape to death. In the original novel, Voldemort was afraid of Snape's strength. Even if he did not consider the Elder Wand, he might not be able to kill him in a one-on-one PK. So Voldemort and Nagini cooperated. Voldemort waved his wand to distract Snape, Nagini covered Snape with a magical cage, and Nagini's fangs sank into Snape's neck, killing him. Harry Potter and the Order of the Phoenix is not enough. Please click to read the novel!
Perhaps Gohan's own overconfidence could lead to his death. He might underestimate an enemy's power or a dangerous situation and make a wrong move that results in his end. For example, he could rush into a fight without proper preparation thinking he can handle it easily.
Maybe it's a group of Hogwarts' own staff members. They could be jealous of Snape's influence or knowledge. They might fabricate evidence against him to get him removed. For example, some teachers who have always disagreed with his teaching style or his past associations.