Given the implications we get from a few episodes (such as "Ed, Pass It On"), it's relatively easy to piece together that Eddy's Brother was a brutal bully to him, so why would Eddy think that such a person would protect him? Was he that desperate, or did he know what his brother would do and pulled a Wounded Gazelle Gambit to get the kids' forgiveness and acceptance?
Eddy still brags of his brother so it's probably a warped relationship since his brother sent postcards and even a large package, if albeit only to make fun of him. This makes things more intriguing.
Further complicating things was that Eddy's Brother did have some implied Pet the Dog moments throughout the series, like teaching Eddy how to make the El Mongo Stink Bomb, the aforementioned post cards, giving Eddy his Porn Stash, and Eddy's claim that he planned on giving him his car after her turned 14.
Eddy could also have been lying because he wanted to pretend he had a good relatioship with his brother, which just makes it sadder.
Ass Pull: True, Eddy's brother's sociopathic nature was foreshadowed a few times in the original show, but the ending feels like this to some. All of the damage Eddy took was all of a sudden Played for Drama, and the kids instantly forgive and befriend the Eds afterwards.
Part of the reason that the kids changed sides so quickly is likely due to the fact that Eddy's brother is a full grown adult, and a family member no less, who is asaulting a child.
Author's Saving Throw: The movie addresses a few complaints about the show: Kevin and Sarah getting hit by karma, Kankers helping the Eds for once, Eddy apologizing for his actions and Eds becoming accepted by the community.
Broken Base: Jonny and his derailment into a joke villain have been met with plenty mixed results.
Harsher in Hindsight: The general response you'll likely get about this movie if you haven't seen it yet is "only watch it if you're done watching any and all of the previous episodes of the series for good. It's the greatest movie that made the entire series up to that point hard to watch now." The movie is often regarded as the highest point in the series, but makes a lot of the older episodes really hard to watch, especially when the Eds suffer Disproportionate Retribution from the rest of the kids in the Cul-da-Sac. When it says it's the Grand Finale, it means it.
Jerkass Woobie: Eddy, after Edd briefly ditches him. He does become a full-fledged woobie after his brother beats him up and reveals that his jerkass behavior was all a facade.
Misaimed Fandom: Some people enjoyed Eddy getting beaten by his brother because of the quicksand prank. Others at least decided when he's had enough.
Moral Event Horizon: Eddy's Brother crosses it when he begins to hurt Eddy, and then admits freely that he's always done this to him.
One-Scene Wonder: Eddy's brother who only appears in the last five minutes of the movie yet leaves a huge impression.
Rescued from the Scrappy Heap: Kankers try to help the Eds and are noticeably disgusted and terrified when Eddy and Edd get beaten by you-know-who. Contrast this with Fistful of Ed, when they weren't above abusing curb-stomped Double D. Contrast this with everything else related to them.
Sarah and Kevin's Karma Houdini Warranty expires and, at the end, finally accept the Eds. Kevin even offers them some of his jawbreakers.
Jonny. The ending of the movie was happy and heartwarming for everyone...except for Jonny who is beaten up by the kids, left without friends and he even turns evil. All because of a misunderstanding.
Eddy. After learning that his brother frequently beat the living daylights out of him, prompting him to create a Jerkass Fašade as a desperate attempt to make friends - then recounting various episodes where Eddy is punished and humiliated... who knows what would have become of him were it not for Ed and Double D's companionship...
What Could Have Been: Many scenes from the movie were deleted and cut down to save time. Most prominently, the ending: Originally, the ending was supposed to have Eddy realizing Ed and Double D were the brothers he's always wished to have and turning on his own brother.