Although his origins have been retconned a few times, Bizarro remains one of Superman's toughest villains. He is the flawed opposite of the Man of Steel, as powerful but usually with far less intelligence. Recently, the character was resurrected and appears to be much smarter.
Bizarro's abilities are generally opposite to Superman's, too. Whereas Superman has heat vision and freeze breath, Bizarro has freeze vision and flame breath. Their weaknesses are different, too, as Bizarro is actually strengthened by Green Kryptonite. Though he has flirted with being a hero at times, Bizarro has committed some truly terrible acts.
10 Froze Killer Croc
Frequently, Bizarro is depicted as a lovable dimwit who isn't necessarily evil but does bad things because he doesn't know any better. In one such instance, he was fighting bad guys alongside his friend, Red Hood. Reinforcements in the form of Killer Croc arrived, who got the drop on Red Hood.
The villain's attack sent Jason Todd flying across the room, but he was saved by Bizarro. Things went downhill from there as Bizarro flew into a rage, froze Killer Croc's head, and tried to smash it. Red Hood stopped him and later found out that this Croc was an android. Bizarro was just trying to help his friend but in the worst way.
9 Lost Control
The Killer Croc fight actually started with Red Hood offering a roomful of thugs a chance to walk away and turn their lives around. Not surprisingly, they did not accept Todd's generous offer as they felt they had the numbers advantage. While that was true, Bizarro more than made up the difference.
Red Hood and Bizarro took down the entire room, which led to the android version of Killer Croc being sent in. During the thug fight, Red Hood had to prevent Bizarro from killing some of them. The aftermath of all of this was Jason Todd briefly considering killing Bizarro to protect the world, an idea he stepped back from.
8 Kidnapped Lois Lane
This one wasn't so much a one-time thing as it was an ongoing situation. Several, if not most, incarnations of Bizarro were imperfect clones of Superman. As such, they also often ended up with the same romantic feelings towards Lois Lane.
As one would expect, these amorous feelings would frequently end up with Lois being the subject of kidnapping and Superman having to rescue her. This let up for a while when Bizarro was able to create his own version of Lois and live happily with her for a time.
7 Kidnapped A Baby
Bizarro's fascination with Superman is never-ending and not particularly surprising considering he's often depicted as a clone of the Man of Steel. But he took this obsession to new levels when he tried to recreate Superman's arrival on Earth in reverse. Bizarro stole the baby daughter of Lex Luthor and attempted to launch her into space using a pod he perceived as similar to the one that brought Superman to Earth.
As insane as the plan was, the method of propulsion would have launched the incident from kidnapping to murder. He was literally just going to use a pile of explosives to fire the baby into space. Fortunately, Bizarro was stopped before anything really bad happened.
6 Tried To Kill Gotham's Henchmen
After being resurrected by Lex Luthor, Bizarro came back smarter and more dangerous than ever. Like Patrick Starfish or Homer Simpson, going from dumb to smart might have been more trouble than it was worth.
But Bizarro came up with a plan to deal with all of Gotham's henchmen. He lured as many of them as possible to a meeting where he asked them to change their ways. They refused, and he started pumping the room full of poison gas. While he stopped short of finishing the job, his intention was to kill hundreds of people in one shot.
5 Killed A Lot Of Terrorists
Bizarro might have been inflicting punishment on the bad guys but there were still other ways to handle the situation like this. In a battle in Qurac, terrorists opened fire on Red Hood and Artemis. Always one to protect his friends, Bizarro stepped in to help them.
While his intentions were relatively pure, his methodology wasn't and he killed all of the terrorists. Just to make sure he got his point across, he then destroyed a mountain with one punch. Once again, Bizarro let his rage get the better of him.
4 Killed A Scientist
As they became more and more concerned about their friend, both Red Hood and Artemis started mulling over the idea of killing Bizarro. Keeping in mind they ultimately didn't, they were rightfully worried about the level of danger a being with that much power and that little control presented.
The duo reviewed tapes of the scientists who created Bizarro, which is how they discovered there were multiple clones. One of the worst incidents on the tape was their Bizarro killing a scientist, possibly for no reason other than he could.
3 Killed A Security Guard
Lex Luthor has often been the person who commissioned the creation of Bizarro, presumably as a part of various plans to destroy Superman. This time, he brought Bizarro to life to help fight the Crime Syndicate of America.
But as part of what was likely a test of control, Lex ordered Bizarro to kill a security guard. He seemed conflicted at first but obliged his creator when the security guard shot him. The bullet didn't hurt Bizarro but it did set him off.
2 Killed Human Bomb
During Infinite Crisis, Bizarro was tricked by Professor Zoom into joining the Secret Society of Super Villains. As Bizarro was not inherently evil, other DC villains would often get him to join them through deceit and manipulation. While on Zoom's team, he encountered a hero by the name of Human Bomb.
Bizarro became fascinated with the light show Human Bomb could put on while fighting him. Wanting the good times to continue, Bizarro started beating Human Bomb in order to get him to produce more pretty lights. He was beaten to death by Bizarro before that could happen.
1 Destroyed Bizarro World
In the Alan Moore story, "Whatever Happened to the Man of Tomorrow?" a young journalist was interviewing an older Lois Lane. The subject was the last days of Superman, who had been missing for years and presumed dead.
By the end of his final battle, several of his closest friends and greatest enemies lay dead around him. But Bizarro also made an appearance in the story after destroying his home, Bizarro World. It was a place he constructed filled with other Bizarro clones. The number of life forms that would have been killed is almost unfathomable.