The protagonist sees his female childhood friend hugging his male childhood friend on the rooftop of school.
The heart of the protagonist then burns with uncontrollable jealousy. He feels himself betrayed by his female childhood friend, since he has finally noticed that he loves her very much.
The protagonist then begins a revenge strategy, a revenge plan. He wants to hurt the girl, hurt her deeply. He will do whatever that is necessary to hurt the girl and damage her in the same way, or more, that he is feeling damaged now.