The Beauty: Ezerae states that kids can be assholes, while showing a picture of her getting beaten up on the playground as a kid (and as a boy).
Cletus Kassidy started out in this trope and got worse from there. To wit, he killed his dog, his family and apparently burnt down the orphanage he was sent to before he became a teen.
Robin: Damian Wayne is currently the youngest member of the Batfamily and one of its least pleasant members with his superiority complex and Anti-Hero tendencies. Snobbish, self-entitled, and manipulative, he constantly rubs others the wrong way and it took a while for him to get over his "killing is okay" mindset instilled in him by his mother, Talia al Ghul. Nevertheless, he still has his moments, such as the time he dropped the younger and naive Jonathan Kent off an eighty story building just to distract Lex Luthor with full knowledge that he was unable to fly.
In Runaways (2015), all the horrors of the Doom Institute were orchestrated by Valeria von Doom, God-Emperor Doom's brilliant but sadistic four-year-old daughter.
On the same note, the first villain of Super Sons is a boy calling himself Kid Amazo. After he and the rest of his family gained superpowers through the Amazo Virus, they fell into in-fighting that ended with Kid Amazo murdering his entire family out of the scorn he believed they had for him.
This plays into a notable event in Rorschach's past in Watchmen. Rorschach was the son of a prostitute who often beat him. One day, when he was ten years old and sent out to get some milk, two fifteen-year-old boys accosted him and kept up the taunt of "whoreson" and demanded to be hooked up with Rorschach's mother. Rorschach, at age ten, partially blinded one boy with the boy's own cigarette, and swarmed the other one. Rorschach was taken into custody, and then put in state school when his home life was discovered.
The X-Men in every incarnation displays this trope. X-Men being what they are, this is liable to fall under Bullying a Dragon.
Young Avengers: Wiccan's backstory is this mixed with Beware the Quiet Ones; when a bully was beating him up for intervening, he accidentally electrocuted the boy, discovering his powers for the first time.
The Plutonian, from Irredeemable, was forced to put up with bullying, as a child, because any retaliation could have killed the kids bullying him. After his Face–Heel Turn, he uses that to justify his willingness to murder children.
