Throughout most of the Kirby games, Kirby ends up doing things that ultimately wind up freeing or unleashing the Final Bosses. Usually, I just figured that this is because Kirby is too selfish to realize just what his actions do, but after thinking about it, I realized that in most cases, the character he defeats (usually Dedede) takes a morally gray option to avoid the boss, while Kirby has an extreme case of Black and White Morality. In addition, when a player reaches that point for the first time, he only knows about as much as Kirby himself does, and since Kirby sees what Dedede does as a bad thing, so does the player. And since the situation changes at times, Kirby's trusting nature and Genre Blindness prevents him from knowing what evil is hidden just out of sight.
In the intro to Kirby and the Amazing Mirror, you see Meta Knight fight his evil clone Dark Meta Knight in the Mirror Word. Meta Knight successfully defeats Dark Meta Knight and imprisons him in the Amazing Mirror that acts as a passage between Dream Land and the Mirror World. Cut to Kirby minding his own business, when suddenly Meta Knight appears out of nowhere and attacks Kirby, splitting him into four! That's a bit our of character for him, but you pay it no mind as you're soon thrust into the Mirror World, looking for the pieces of the Amazing Mirror so you can return to Dream Land. The way forward, however, is never what it seems to be, and to reach the Mirror Shards you have to search every nook and cranny for secrets and hidden passages. So you face Meta Knight, who naturally doesn't want you to restore the Mirror since it would free Dark Meta Knight, and... Wait, what's this? He's not giving you a sword? That's definitely out of character. Something fishy is going on here... So you find the remaining pieces of the Mirror, and finally come face to face with Meta Knight inside... And then the real Meta Knight appears! The Meta Knight you've been fighting is actually Dark Meta Knight , and the Meta Knight that was imprisoned was the real one all along! After knocking the crap out of Dark Meta Knight, he reveals his true form: an evil sorcerer known as Dark Mind with an appearance similar to Nightmare. Look closely, however, and you'll see a fiery eyeball where his torso should be. Strange... After defeating Dark Mind a few times in a row, he then reveals his *actual* true form, the same fiery eyeball that he had for a torso, only much larger! Even after you finally reveal his true self, he still tries to throw you off with tricky tactics, such as reflecting his shots off of mirrors and even flipping the screen upside-down! So after you finally defeat Dark Mind, he explodes as the four Kirbies fly off back toward Dream Land. ...And then it's revealed that the Shadow Kirby enemy you've been beating up the whole game is *not* a bad guy like you probably assumed, but is actually the hero of the Mirror World! Every aspect of the game, from the story, to the presentation, to the metroidvania gameplay, is representative of a central theme: Kirby and the Amazing Mirror is a game about exposing lies.
The Kirby series is probably the archetypal Sugar Bowl, a perpetually happy place filled with bright colours and cheerful characters where nothing bad ever happens, the stories merely Excuse Plots for purposes of fun. Except that no, it isn't. Pop Star is constantly assailed by undefeatableEldritch Abominations that corrupt and kill and bring the world to the brink of destruction. The closest thing to an authority figure is a bumbling Authority In Name Only that can barely succeed even in the few times he manages to be competent. The Only Sane Man is a Blood Knight concerned only with combat. And Kirby himself, the protagonist and supposed hero, eats people. Hundreds upon hundreds of people. He is a horrific scale Big Eater on par with the Tyranids, consuming indiscriminately all in his path, and there is nothing anyone can do to stop him.
Keep in mind that most of these "enemies" are completely innocent. Most of them just sit there until Kirby either devours them or destroys their souls.
Also keep in mind he doesn't actually kill them, as you can later spit them out in Super Star.