Fred was dating Angelina Johnson, right? That's what he thought. Until he caught his own twin brother, George, kissing her. After that, it got worse. George and Angelina had already been talking and hugging and holding hands a lot, which made Fred insecure of his ability to hold a girl. The kiss was the last straw, and he started disliking George and Angelina both since.

They continued getting closer, and sometimes would even mock Fred about their relationship. Fred had enough, and resorted to physical violence to shut them up. It only worked well for a week. After that, they weren't so phased by it, and hit him, as well.

As the twins grew further apart, the Weasleys and even Lee Jordan took George's side. That was the last straw. Soon, Fred became friends with the Slytherins, and he even joined the Death Eaters after his entire family and friend group hated him for being "overdramatic".

das basically it lol :3