There are few characters more lovable in the Marvel Cinematic Universe than Groot. The tree-like hero is the heart of the Guardians of the Galaxy, as his friendship with Rocket Racoon is one of the best in both the comics and films. Groot wasn't always a good guy, though. In his first appearance in the comics, he was a full-fledged villain.
Groot has had one heck of a ride so far in the MCU. Originally at full-size, Groot sacrificed himself to save his friends at the end of Guardians of the Galaxy. He's regrown in the Guardians of the Galaxy Vol. 2, where he becomes Baby Groot, who unlike his previous incarnation is very misbehaved. In Avengers Infinity War and Avengers: Endgame, Groot is a teen who's got serious attitude issues. However, Groot is later vital in creating Thor's weapon Stormbreaker. After getting snapped out of existence by Thanos, Groot returned with fellow snapped heroes for the final battle against the Mad Titan.
Groot's first comic appearance came back in 1960 in Tales of Suspense #13 by Stan Lee, Larry Lieber, and Jack Kirby. Instead of the lovable character that could only say "I am Groot," the original version of the hero was a villain who proved to be quite the menace for humans on Earth. In the story, "the monster from Planet X" arrives and starts to absorb the trees surround him. He tells the humans that he's a "Monarch of Planet X" who wants to take an Earth village back to his home so they can be studied and experimented on. Groot threatens the scared humans by telling them he can enclose their city in trees and use the roots to carry the entire village back to his home planet.
As weapons prove useless against Groot, he commands the trees as roots start to overtake the city. Thankfully, a scientist manages to stop Groot by developing his own experiment, releasing a group of termites that he bred in his laboratory. Groot would later be brought back in Annihilation: Conquest, who was more like the version known in the movies today. But, the original Groot was unquestionably a bad dude.
It's funny how characters can evolve over time. Groot is without question a hero in today's Marvel Comics and movies. However, when he first appeared he was a villain who wanted to experiment on humans. It's quite a significant change - but that's the beauty of relatively unheard characters, as they can be changed for a modern audience in big ways. In this case, Groot works much better as a hero than a villain.