Hal Jordan (the Green Lantern) and Barry Allen (the Flash) are best buddies. It makes sense that they would be, because they’ve got a whole lot in common. For example, they each destroyed the universe.
When Hal Jordan Erased the Universe
The success of the Knightfall arc, where Bruce Wayne was temporarily put out of action and replaced by a more violent Batman, and The Death of Superman, when Superman was temporarily killed off and replaced by four different heroes each vying to wear the iconic S, led DC to explore the idea of replacing some of their other characters as well. The Green Lantern wasn’t a high-performing book, and DC decided to have a younger, hipper hero take up the glowing ring. But rather than kill Hal Jordan off, they turned him into a villain.
Hal’s descent into madness sprang out of The Death to Superman, when the cyborg Superman and Mongul destroyed Hal’s hometown of Coast City. In the pages of his own comic, he used his ring to recreate the city, but ran out of energy after 24 hours (as Green Lantern rings were wont to do in those days). But then the Guardians of the Universe, rather than sympathizing with him and noting that he had just barely saved the planet, chose to punish him for using his ring for personal gain (and yet, strangely, they never had any problem with him creating a giant city-smashing monster just to dodge a marriage proposal). This pushed Hal over the edge, and instead of reporting to the Guardians for punishment, he instead decided to steal all the power in the Central Power Battery so he could resurrect Coast City.
Hal’s transformation from world-saving hero to complete and total monster took all of three issues because DC wanted the new Green Lantern rolled out as soon as possible. The first issue of the Emerald Twilight arc saw him as a grieving hero pushed over the edge, the second issue had him killing multiple Green Lanterns on his way to Oa, and the third issue had him kill all the Guardians except one. But at least his death toll had only been in the thousands rather than the billions, right?
In the Zero Hour crossover a few years later, Hal decided that just fixing up Coast City wasn’t enough…he had to fix the entire world. In order to do that, he destroyed the universe and then made a new one in its place. The Justice League got involved and stopped him before he could really “fix” things, though, and the end result of his attempted reboot of the universe was just a couple minor continuity cleanups and the fact that Superman and Batman were about ten years younger because DC wanted their cash cows to be relatable to an adolescent audience.
Ultimately, the death toll for Hal’s little rampage numbered in the hundreds of billions, but most people were brought back to life in his attempted remaking of the universe. One alternate version of Batgirl got written out of existence, though, and that caused Hal’s buddy Green Arrow to shoot him in the chest. (Hal got better, though.)
How did the heroes of the DC Universe ever forgive Hal? About a decade later it was revealed that everything he did since the destruction of Coast City had been a result of the giant yellow fear-bug that had secretly lived inside him. Because comics, that’s why.
Barry Allen Saves his Mother, Destroys History
In Flashpoint, Barry Allen found himself in a strange alternate universe where the superheroes he knew were broken in some way – Superman had been held in a government facility for decades, Batman was actually Thomas Wayne, who turned into a lethal vigilante after the death of his son Bruce, Wonder Woman and Aquaman were engaged in a war that destroyed most of Europe, and so on.
For most of this event, it seemed like the culprit behind all these changes was Professor Zoom, the Reverse Flash. That guy had the ability to run through time because quantum physics apparently took a vacation in the DC Universe, and he had an unhealthy agenda against Barry Allen. In actuality, Zoom had attempted to kill Barry’s mother and Barry himself had accidentally rewritten history in an attempt to stop that from happening.
Eventually, the Flash was able to save the day…from the catastrophe he created.
The end result of this event was “The New 52,” which was a complete relaunch of all DC’s superhero lines. Most of the new universe was recognizable, except that a lot of the fun Silver Age stuff has now been written out to be replaced by the darker style of comics that DC’s editorial staff preferred. Other than that, a lot of superheroes were written as younger, since both DC and Marvel have decided that apparently nobody wants to read a comic about a superhero who might turn 40 someday.
There were, of course, casualties, with some characters apparently being written out of existence. Certain characters, such as the original Question, Barry’s sidekick and successor Wally West, and more, were either dramatically changed or flat out gone until DC re-re-rebooted the universe with their Convergence event.
There’s also the fact that Barry’s running through time collapsed three different universes: the DC Universe, the Vertigo Universe, and the Wildstorm Universe. Now, each of those had an Earth that had roughly seven billion people on it. And the New 52 Earth apparently only had about seven billion on it. That means that, unless every person on those alternate Earths had a duplicate that they were somehow merged with (unlikely, since, for example, there was no Vertigo Superman or DC Midnighter), that leaves Barry accidentally responsible for multiple billions of deaths.
Since Green Arrow was so upset about an alternate Batgirl getting created and then killed off, I fully expect him to show up one day and shoot the Flash in the chest about 14 billion times. Or at least I would expect that to happen, except that nobody remembers the time that Flash destroyed history and then squashed three universes together. Because the best way to get out of manslaughter on a omniversal scale is to make sure that nobody, including yourself, remembers it.