These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Complete Monster: In his Pre-Crisis and Post-Crisis incarnations (but not, at least as of yet, his New 52 one). When you force Silver Age Superman, perhaps DC's ultimate boy scout, to resort to lethal force, you know you are a monster. His crimes over the years have ranged from shrinking cities and planets for his private collection, to coldly slaughtering thousands in a quest to destroy the "Master Programmer" and become God, to brainjacking Luthor, leading to the deaths of Lex, Lana Lang, and Jimmy Olsen in Whatever Happened to The Man of Tomorrow?. And that's just Pre-Crisis. Since then, he has fed on human spinal fluid, possessed countless victims and destroyed their minds in the process, exploited the ImperiexWar in an attempt to rewrite the fabric of the universe, tried to destroy New Krypton, and, most recently, gone in for planetary destruction and city collection in a big way. He has never shown any remorse, his presence in a story inevitably signals that the bodycount is about to rise, and he has been a genuine physical threat to Superman since the early days when his Deflector Shields made him untouchable.
In Superman Unbound Brainiac is a Control Freak and genocidal psychopath on a galactic scale. Brainiac seeks knowledge and feels the best to do that is to shrink and then steal a major city from a planet and then destroy the planet. He stole Kandor from Krypton (he didn't bother blowing up Krypton because he knew that Krypton would soon be destroyed by natural causes). Then he attacks another planet, again shrinking and stealing a city and then uses a device that causes the planet's Sun to go Nova, thus killing the remaining inhabitants on the planet. Brainiac also sends his drones to the planets he is attacking and the drones proceed to kill random inhabitants of the planet. He later steals Metropolis and tries to destroy Earth in a similar manner. He also tortures Superman for information and tries to experiment on Supergirl.
In Smallville, Brainiac has no emotions and is fond of skewering people through the head and draining the info from their brains. Despite being nearly equal to Clark in power, he prefers to perform complex manipulations to make others do his dirty work for him (including infecting Mrs. Kent with a deadly disease just to trick Clark into releasing General Zod from the Phantom Zone), putting Lana in a coma to force Clark and Kara to help him, giving Clark's secret to Lex, bodyjacking Chloe and using her as part of a plot to brainwash Doomsday, and trying on three separate occasions to Kill All Humans via deadly viruses. In the Wonderful Life episode, without Clark to stop him, Brainiac triggers a nuclear holocaust, saying the world is now perfect for Zod, Zod's consort Supergirl, and himself to rule. And that's not getting into his cannibalism of the silicon in peoples' bodies when he needs to rebuild himself, or his condescending personality, or the fact that Bizarro, Lex, and the various other villains who appear are all disgusted by him. He eventually Heel Face Turns, but since this is due to being reprogrammed and not because of a moral choice on his part, it doesn't count.