Spoilers ahead for Episode 2 of The Flash Season 6, called "A Flash of the Lightning."
Although the Arrow-verse's "Crisis on Infinite Earths" mega crossover is still nearly two months away from premiering on The CW, The Flash has already taken big steps toward setting up the arrival of The Anti-Monitor to potentially destroy the multiverse. The Season 6 premiere of The Flash saw The Monitor drop by STAR Labs to tell Barry that "The Flash must die" for the multiverse to survive, which raised the question of whether the Arrow-verse would kill off both Barry Allen and Arrow's Oliver Queen. After the events of "A Flash of the Lightning," the bigger question is: could the Arrow-verse kill Barry instead of Oliver Queen as Green Arrow?
Let's break it down. Oliver ended Arrow Season 7 determined to die in the Crisis if that's what it took to spare Barry and Kara, as well as protect his family, whereas Barry handled the Flash Season 6 premiere news that he'd die by declaring that he wasn't going to let it happen. In "A Flash of the Lightning," however, Barry's attempts to get a sneak peek at the Crisis resulted in him realizing that this wasn't a matter of being able to survive by just trying really, really hard.
After discovering that he was blocked by antimatter while trying to race to the future, he dropped by Earth-3 to visit Jay Garrick, who had developed technology that would allow a speedster in his prime to project his consciousness where his body couldn't go.
So, Barry was strapped in and sent on a terrifying and trippy mental journey that revealed worlds being destroyed in the Crisis, and The Monitor's words that "The Flash must die" echoed in his head. In case that wasn't enough, he witnessed the deaths of Cisco, Caitlin, and Iris.
Finally, he saw himself racing so fast that he was literally ripped apart, but it was apparently enough for the multiverse to survive, if not Barry himself. He saw billions of possible futures (which probably sounded familiar to MCU fans) and only one ended with the multiverse making it. That one is a future in which The Flash dies.
After going back and forth on whether accepting his demise to save the lives of billions was giving up or noble sacrifice, Barry realized thanks to a heart-to-heart with Joe that he would have to be resilient as a hero, and in the case of this Crisis, this meant running his final race before perishing. (According to The Monitor, anyway.)
Basically, this episode proved that The Flash isn't just going to force Barry to take The Monitor at his word that he's going to die. Barry has seen it for himself, and it shook him to his core. Like Oliver, Barry has accepted that he'll have to give his life for the greater good of defending the multiverse against The Anti-Monitor.
So, why does this mean that Barry could die and Oliver could survive? Well, The Monitor is being cagy when dealing with the heroes of the Arrow-verse, as Barry doesn't know about Oliver's deal with The Monitor and The Monitor probably isn't sharing every detail of his plan with Oliver, deal or not deal.
Barry has also seen the future in which he dies, and seen himself breaking apart as he ran impossibly fast. Oliver just has The Monitor's word that he'll bite the dust. Then, there's the fact that we already know that Oliver doesn't actually die in "Crisis on Infinite Earths," even though it's been teased as "devastating."
The Arrow Season 7 finale revealed that Oliver's friends, family, and loved ones on Earth-1 seemingly believe that he died in 2019, but The Monitor whisked flash-forward Felicity away to reunite her with her husband after a long separation. That was in 2040. So far, we have seen Barry die, and we have seen proof that Oliver survives. Throw in the fact that The Monitor isn't being altogether forthcoming, and it stands to reason that The Flash will indeed die while the Green Arrow survives, even if the circumstances demand that Oliver fake his death on Earth-1 and spend the next 20 or so years elsewhere.
Of course, we have to account for the fact that Arrow is ending while The Flash presumably is not, and The Flash doesn't have a speedster set to step into the scarlet cowl right off the bat if Barry does die in "Crisis on Infinite Earths." Fans of DC Comics know that Barry Allen's Flash was gone after "Crisis on Infinite Earths," and his protege Wally West took on the Flash mantle. Arrow's Wally West left the action a while ago, Jesse Quick doesn't seem to be in the equation, and I doubt Reverse-Flash is going to drop the "Reverse" and become the hero of Central City.
DC Comics fans also know that Barry Allen does ultimately return as The Flash, so The Flash could totally kill off Barry Allen as The Flash, spare Oliver but send him into secret exile for a couple of decades, and leave the Arrow-verse devastated at the loss of its two longest-running heroes... until the time comes for Barry to return via some Speed Force shenanigans or time travel or some other impossibility that The Flash pulls off on a semi-regular basis. This would ruin my theory about the evil Batman saving the day, but I could live with that!
Unfortunately, "Crisis on Infinite Earths" doesn't kick off until December, so fans won't get their hard answers for a while. In fact, if we assume that the Arrow-verse is killing off a big-name hero or two, then the odds are pretty good that it wouldn't happen until the second two parts of the five-episode crossover air in January. Still, no matter what happens and no matter how devastating "Crisis" may be, there should be plenty to entertain longtime Arrow-verse and longtime superhero fans.
"Crisis on Infinite Earths" is bringing together the heroes of all five Arrow-verse shows for the first (and only, thanks to the coming end of Arrow) time, as well as bringing in Smallville veterans, a Batman voice actor in the flesh rather than animation, a former Boy Wonder, and maybe even the Devil himself, to name only some of the cameos.
For now, you can catch new episodes of The Flash and Arrow on The CW Tuesdays, starting at 8 p.m. ET with the adventures of the Scarlet Speedster and continuing at 9 p.m. ET with the efforts of the Green Arrow.