During The '90s, Batman became something of an arrogant rude prick who'd constantly insult and sneer at every other hero just for not being as edgy as he was, and treated the rest of the Batfamily with aloofness and distance. He's gotten better... sorta.
Robin (1993): Near the end of his career as Robin, Tim Drake's personality took a dark turn, even using Scarecrow's fear toxin against gangsters. The reason for this was largely due to losing his mom, dad, girlfriend, best friend, other best friend, stepmother, and adoptive father in short order and then having Dick, Stephanie and Cassie all betray his trust. Mostly Dick who refused to even listen to Tim's theory about what had really happened to Bruce and instead decided Tim was losing his mind, culminating in Dick giving Robin to Damian, who had made repeated attempts on Tim's life.
In Batgirl and the Birds of Prey, Huntress appears to be Darker and Edgier than she was in Grayson, as she's shown to be killing every criminal that crosses her path, even threatening both Batgirl and Black Canary. She does mellow down by the end of the first arc, however.
The original Eric Needham Black Spider was this after his Unexplained Recovery as during Identity Crisis (2004), he was shown willing to hang out with the very villains he originally swore to take down.
As a result of the trauma his family suffered in The Joker War, Lucius Fox underwent this in The Next Batman: Second Son, turning his back on Bruce and becoming more willing to take questionable steps to keep Gotham safe, including going down the path to help turn Gotham into the Police State seen in DC Future State.
After her husband's death, Alura In-Ze becomes harsher, more callous, more unreasonable and more unpleasant. She snaps at everyone and becomes emotionally abusive to her own daughter. In Who is Superwoman? she treats Kara as a reckless, undisciplined kid or an incompetent, useless soldier.
Kara: Lana, what's wrong with her? Why is she so horrible to me? I do what she asks. I try so hard to help her, and all I get is her anger. Her disapproval. She wasn't like this when dad was alive.
The Phantom Zone: Jax-Ur had never been good, but he was not a sadistic criminal, either. In this story he is turned into the kind of psychopath who enjoys snapping bones and slamming people through walls.
Power Girl becomes real unpleasant and mean-spirited in Girl Power when she rejects Supergirl's sincere offer of friendship because she is upset at no longer being Superman's only cousin.
In Superman: Doomed, the Doomsday virus causes Superman to snap at friend and foe alike.
In Superman: Grounded, Dick Grayson, of all people, has taken one when he becomes the new Batman after Bruce Wayne's was lost in time. He tells Superman he shouldn't become close to the people he protects, otherwise he risks making them targets for his enemies. Remember, this is the same Dick Grayson who is known for being nicer and more socially-adept than Bruce.
Although Dick Malverne had previously suspected Linda Danvers was Supergirl, he no longer cared either way when they began dating. In The Girl with the X-Ray Mind, he tricks himself into believing Lena Thorul is Supergirl, and he suddenly starts completely ignoring his girlfriend while paying attention to and trying to "expose" Lena. The retcon introduced in Young Love, according to which Dick was never fooled by Linda's tricks, makes him look even worse.
In DC Rebirth continuity, Superman's father Jor-El can be described as this. Instead of being a benevolent, humble figure, he is a Well-Intentioned Extremist involved in much more shady, underhanded and outright dangerous activities. Jor-El was part of a cabal of six secret rulers from different planets who ruled their galaxy from the shadows and met in secret, and he would later create the humanoid behemoth Rogol Zaar to act as a mercenary for the Circle and carry out their dirty work in the galaxy. He becomes worse when Dr. Manhattan, the catalyst behind the previous New 52 universe who also set in a chain of events leading to the Rebirth universe, kidnapped him at the second Krypton was about to explode and placed him in the most war-torn city on Earth. After he saw the evils of some human men, he remarked how it was unbelievable for someone to be so wicked as to deny another human food. Later, Dr. Manhattan locks him in a room with video monitors where he witnesses man's inhumanity to man across generations. This broke him and made him feel like humanity did not deserve his son's protection. He adopted the moniker of "Mr. Oz." and manipulated humanity's deepest emotions of hatred so they would turn against each other. At the end, as punishment for his many sins and repeatedly violating the laws of nature and science, the Intergalactic Council returns him to his original place in the timestream, of Krypton's destruction, so he would perish along with the rest of the Kryptonians.
Green Lantern: Guy Gardner was a fairly regular man. That is until he was hit by a bus, got trapped in the Phantom Zone where he was tortured by General Zod, and ended up in a coma upon his return to earth, only to wake up years later with a massive chip on his shoulders.
In The Just #1, the older heroes of Earth-16 have no issue with re-enacting major battles for fun, regardless that some of their closest friends and allies actually died in those battles. For example, they have Red Amazo (the combination of Red Tornado and Amazo) participate in the recreation of the event that actually created him, but pretty much killed the Red Tornado in the process.
After Jenny Sparks died, The Authority as a whole did this: lethal force was no longer merely an option, but their only solution to dealing with their problems; they often acted unilaterally without regard for the consequences for their actions or the concern of others; and for a brief period, they took over the United States.
The Sandman (1989): While Dr. Destiny was always a villain who crossed paths with the Justice League of America, he never engaged in the behavior he did in the first arc, which infamously included torturing people in a diner for a day for his own amusement and driving a kid's show host to commit suicide on live TV — and encourage his audience to do likewise.
