SPOILER WARNING: This article contains major spoilers for “Crisis on Earth-X, Part 4,” the fourth episode of this year’s Arrowverse crossover.
In the first two parts of "Crisis on Earth-X," this year's Arrowverse crossover, Dark Flash revealed that he is actually Earth-1's Eobard Thawne, despite his death in DC's Legends of Tomorrow Season 2. As such, he and Barry have a long history, stemming from Thawne murdering Barry's mother Nora when he was just a child. Now, Thawne is loose on Earth-1 again, free to terrorize Central City and Team Flash as he pleases.
After zipping all over Central City in a climactic battle, Barry over came Thawne and slammed him up against a concrete wall. Barry began to vibrate his hand, in the same way Thawne does when he intends to kill, but Barry hesitated before delivering the fatal blow.
"Go on! End it!" Thawne hissed. When Barry continued to stall, Thawne yelled, "End it!"
After a moment, Barry stopped vibrating and lowered his hand. "What's stopping you?" Thawne inquired. "Ah, I forgot! Barry Allen is above killing. Isn't that right?"
Barry stepped back, freeing Thawne from his grip. "Get out of here," he ordered.
Thawne stalked away, only to turn slightly and look back at Barry. "I wonder what face I'll be wearing next time we meet, Flash," he said with a sinister smile.
"Can't wait," Barry spat, and Thawne took off, away from Barry and the fight.
It's unclear why Barry let Thawne go, as he did not have a chance to explain his reasoning. However, in doing so, he has chosen to let the threat of Reverse Flash remain on Earth-1. Seeing as Thawne will stick around, he will likely turn up again sometime, in this season or otherwise.
This year’s four-part crossover “Crisis on Earth-X” will air across two nights, beginning Monday, Nov. 27 with Supergirl and Arrow (on a special night for one week only) and concluding Tuesday, Nov. 28 with The Flash and Legends of Tomorrow.