What are the signs of McGonagall hating Harry in fanfic?

One possible reason could be that she is under some sort of dark magic influence. Maybe a spell has been cast on her that makes her act out of character and abandon Harry.

She might use her magic skills to shield Harry from physical harm. For example, she could cast protective spells like a strong shield charm when Harry is in danger from dark wizards. Also, she may use her authority in Hogwarts to keep him away from those who wish to harm him, like making sure he is not alone in dangerous areas of the school.

Perhaps when Harry is wrongly accused of something, and McGonagall uses her influence and knowledge to prove his innocence. She stands up for him in front of the whole school or the wizarding authorities, which is a very significant moment of her protecting Harry.

He might avoid any events or communities related to magic fanfic. He won't be interested in reading or discussing them.

One possible plot could be that after some sort of danger at Hogwarts, perhaps a new dark threat, Dumbledore decides that Harry would be safer living with McGonagall. McGonagall, being strict but fair, would impose a strict study regime on Harry. She'd also teach him some advanced magic that he might not learn in regular classes. Harry would get to see a different side of her, like her love for cats which could lead to some comical moments as he tries to interact with her Animagus form.

It could be that in the fanfic's plot, McGonagall has some past connection to Harry's enemies. So, her hatred towards Harry is a result of her loyalty to those she has a history with. This would create a lot of drama and conflict within the fanfic's story.

One reason could be a personal betrayal. Maybe a close friend from Gryffindor betrays Harry, and this makes him start to dislike the whole house. It's like when one bad apple spoils the bunch in his eyes.

Harry might be initially shocked and confused. He has always seen McGonagall as a reliable figure in his life at Hogwarts, so her abandonment would be a huge blow to him.

No, 'The Hating Game' is not a fanfic. It is an original work by Sally Thorne.

Definitely not. The Hating Game has its own unique creation and plotline. It didn't originate from the world of fanfiction.