The consequences would be catastrophic. Firstly, it would completely shatter the wizarding world's social structure. Pureblood families have been an integral part of the wizarding society for centuries. Secondly, it would create a moral vacuum as killing all purebloods is mass murder. And finally, it would go against everything Harry stands for in the original series, leading to a complete distortion of his character.
If such a 'harry kills all purebloods fanfic' were to exist, the consequences would be many. Morally, it would be a very dark and unethical scenario. The wizarding world would lose a large part of its cultural and magical heritage that is associated with pureblood families. Politically, there would be a power vacuum and likely a struggle for control among the remaining magical groups. In terms of relationships, characters like Hermione, who has friends from different blood statuses, would be completely at odds with Harry. And it would make the story a very different and disturbing take on the beloved Harry Potter universe.