It could be Voldemort or one of his Death Eaters. They might have used magic to make it seem like Harry and Snap were the killers. Voldemort has always been cunning and would love to see Harry discredited. He could have used an Imperius Curse on someone to make them plant false evidence or even stage the murder scene to point towards Harry and Snap.
Perhaps there was a miscommunication or a misunderstanding. They could have been trying to protect someone from a real threat, but in the chaos, it looked like they committed murder. Say, they were fighting off a group of Death Eaters and in the crossfire, an innocent bystander was killed, and the Ministry of Magic, being quick to judge, branded them as murderers without fully investigating.
One possible plot could be that Harry and Snap are facing some sort of danger in Britain, perhaps from dark wizards or a corrupt magical government. So they decide to flee. They might embark on a journey to find a safe haven, like a hidden magical community in another country. Along the way, they encounter various challenges and make new friends who help them in their quest.
Well, clearly Harry and Snap are the main characters. Harry is likely a well - known character, perhaps from the Harry Potter universe. As for Snap, not sure if it's a new character created specifically for this fanfic or a less - known character from the existing lore. Maybe Snap is Harry's friend or a mentor - like figure, and together they are the focus of the story as they flee Britain.
In most of the official 'Harry Potter' canon, Vernon Dursley didn't murder anyone. However, in fanfic, the story can take many wild turns. It depends on the specific fanfic in question. Some fanfics might create extreme scenarios where Vernon could be involved in a murder, perhaps out of his extreme dislike for magic or a character related to magic. But without reading the particular 'Harry Potter Vernon murdered' fanfic, it's hard to say for sure.
One possible motive could be his intense hatred for the wizarding world. Since Harry represents that world to him, he might murder someone he believes is part of the magic that he despises so much. Another motive could be fear. If he feels threatened by a magical being or situation, in his panicked state, he could resort to murder.
A pure - blood supremacist who is secretly part of a group trying to rid the wizarding world of Muggle - borns. They could be targeting students or professors who support Muggle - born rights, and their actions are disguised as random murders to avoid suspicion.
It could be that the characterizations weren't right. If the characters act out of character, it can disappoint fans. Also, the writing style or the pacing might not have been appealing.
Harry Potter is a very resourceful wizard. He might first try to understand the nature of the Snap. He could use his invisibility cloak to get close to Thanos unnoticed. Then, he could use the power of the Elder Wand, which is the most powerful wand in the wizarding world. With a well - aimed spell from the Elder Wand, he could potentially break the connection Thanos has with the Infinity Stones and stop the Snap.
A plot could revolve around a series of murders that seem to be related to a long - lost and cursed Hogwarts artifact. As the story unfolds, the main characters discover that different people are being targeted because they are all descendants of those who once had a connection to this artifact. They have to figure out how to break the curse before more people die.