Hagrid is one of Harry's favorite people at Hogwarts and we get to find out a little bit more about his backstory in this movie. Fun fact: He went to school with Voldemort. That would make for some weird yearbook photos.
Okay, let's back up. We found in the first movie that Hagrid wasn't really allowed to do magic, but we never learned why. Now, we get the whole story. Turns out that Hagrid was expelled from Hogwarts because the headmaster at the time actually thought he opened the Chamber of Secrets and killed a girl.
Tom Riddle shows Harry this memory of how it all went down:
TOM RIDDLE: Evening, Hagrid. I'm going to have to turn you in. I don't think you meant it to kill anyone...
RUBEUS HAGRID: You can't. You don't understand.
TOM RIDDLE: The dead girl's parents will be here tomorrow. The least Hogwarts can do is make sure the thing that killed their daughter is slaughtered.
RUBEUS HAGRID: It wasn't him. Aragog never killed no one. Never.
TOM RIDDLE: Monsters don't make good pets, Hagrid.
Basically, this is bad timing for Hagrid. He is harboring a dangerous magical creature—an Acromantula—in the castle at exactly the same time a beast has killed a girl. And this is lucky for Tom Riddle, who has actually opened the Chamber of Secrets and released a basilisk, that he can frame Hagrid to take the fall.
Hagrid's love for dangerous animals has always been a bit of a problem. Yes, he's a sweet and loveable guy who just wants to take care of things, but we're talking a huge man-eating spider that almost killed Harry and Ron. We never thought we'd say this, but we're with Voldemort on this one—monsters do not make good pets.
Okay, so we get why Hagrid was expelled fifty years ago. He was in the wrong place and the wrong time and took the fall for a smooth-talking manipulator like Voldemort. What we don't get is why it took more than fifty years for people to realize that Hagrid was innocent.
Seriously. The Minister of Magic comes and arrests Hagrid and no one thinks to say, "Hey, that evidence might not be so rock solid given that it was presented by a young Voldemort." Isn't that the perfect bit of reasonable doubt? Tom tells Harry that, "It was my word against Hagrid's," but once it was Voldemort's word against Hagrid's then aren't people leaning a little more towards Hagrid? Just a tad.
Did we just shake your faith in the wizard justice system? Well, it won't be the last time. Buckle up for the rest of these movies because it's going to be a bumpy ride filled with many injustices.