WARNING: The following article contains spoilers for The Flash #753, by Joshua Williamson, Howard Porter, Brandon Peterson, Hi-Fi and Steve Wands, on sale now from DC.
First introduced in 1957's Showcase #8, Barry Allen/Flash and Leonard Snart/Captain Cold have had a complicated relationship, to say the least. While the two have previously been begrudging allies, Cold is once again a clear-cut villain in the DC Universe. During the "Rogues Reign" arc, Cold even got a massive power-up from Lex Luthor and Perpetua as part of "DC's Year of the Villain," which enabled him to take over Central City. Now, Paradox -- Barry's newest villain -- has killed Captain Cold.
After defeating the Scarlet Speedster and Godspeed outside of the Flash Museum, Godspeed absconded with the latter and took control of the Cosmic Treadmill, which Barry has used in the past to travel through time. Paradox takes the two back to the climax of "Rogues Reign," in which Snart -- who has taken on the moniker King Cold -- faces off against Barry. In the original timeline, Snart was betrayed by his allies, but his enhanced abilities allowed him to nearly take down Flash, who was still reeling from the "Death of the Speed Force" arc, which made the titular entity unstable. However, Barry triumphed when the malfunctioning Speed Force caused him to turn gold and nearly beat Snart to death.
In The Flash #753, Barry is shocked to see Godspeed travelling in on the Cosmic Treadmill. Snart, however, turns his attention to Paradox and attempts to attack the villain. Before Snart can hit the new villain, though, he's blasted with some of Paradox's temporal energy, which rips him apart. Paradox then turns his attention to Barry and kills the hero. His motivation for this brutal act being that these changes to continuity will allow him to grow more powerful, since Paradox essentially feeds off of the Crisis energy unleashed by such actions.
It's unlikely Captain Cold will stay dead. Barry has now teamed up with Eobard Thawne/Reverse-Flash, who previously defeated Paradox when the villain turned up in the 25th Century. As such, Barry is going to be looking for a way to undo Paradox's actions and restore the integrity of the timeline, especially since these alterations pose a genuine threat to the stability of the multiverse. Still, the act of killing Captain Cold in his empowered state proves Paradox might just be the deadliest villain Barry has ever faced.
The Flash #754 goes on sale May 27.