It could be a new dark wizard we haven't seen in the original series. Maybe someone who has a vendetta against Hogwarts and manipulates Harry into doing the dirty work of destruction.
It could be that there's a powerful enemy that has taken over Hogwarts and Harry, in a desperate attempt to stop this evil from spreading, resorts to destroying the school. He might think that rebuilding it later is better than allowing it to be used for evil purposes. For example, if Voldemort had found a way to permanently taint the magic of Hogwarts, Harry might see destruction as the only option.
One possible reason could be a dark turn in the story. Maybe Harry is under the influence of a dark magic or a powerful and evil entity that makes him lose control and start to destroy Hogwarts. For example, if he was possessed by a powerful dark spirit that wanted to eradicate the symbol of the wizarding world's good, which is Hogwarts.
There very well could be. Fanfiction writers often like to explore unusual crossovers. Since both Harry Potter and Thanos are extremely popular in their respective franchises, it's a ripe area for creative exploration. However, the quality and availability can vary greatly depending on the fanfiction platforms you search on.
There would be shock and disbelief. The wizarding world has always seen Hogwarts as a symbol of hope, knowledge and safety. If Harry destroys it, many would be in denial at first, thinking it couldn't be true. Wizards and witches from all over would come to the ruins, trying to figure out what had happened.
The Order of the Phoenix members might also back Harry. They've fought against the dark forces before and if they see that the Ministry has become a threat, they'd side with Harry. Dumbledore's old allies in the Order would believe in Harry's judgment and join him in his mission to destroy the corrupt Ministry.
Well, it's a very dark and unorthodox concept. In the original series, Dumbledore is a mentor figure to Harry. In fanfiction, if Harry were to destroy Dumbledore, it could signify a major deviation from the core values of the original story. It might be exploring themes of betrayal, a different view of power dynamics, or a radical re - interpretation of the characters' natures.
Probably not. Given that the Weasleys are beloved characters in the Harry Potter universe, a fanfiction with such a negative and extreme concept as Harry destroying them is likely to be off - putting to a large number of fans. Fans usually look for fanfictions that expand on the positive aspects of the story, like friendships and adventures, rather than something so dark and against the norm.
I'm not sure about the specific plot of this fanfiction as it sounds rather unorthodox. It could be a very dark or alternate - universe take on the Harry Potter world where Harry goes against the Weasleys for some extreme reason.
I have no idea who the author of that fanfiction is. There are so many fanfiction writers out there, and without more information it's impossible to determine.