While Red Death's claims of fighting a secretly evil Barry Allen are possible, the audience only has her word for it. Iris is very sceptical, and points out that Red Death's actions are exactly the sort of thing Barry would try to stop anyway.
It's also unclear on whether Barry's cowl is actually booby-trapped, or whether Mark just made that up to help Barry. Given that it is the kind of thing Cisco would do, and that Barry still treats Mark as a villain in the scene afterwards, the implication is the former.
Animated Armor: Similar to Savitar, Red Death can control her armor from afar.
Badass Boast: Red Death lets out one that references Batman's famous quote.
Red Death: You want to know who I am? I'm the last glimmer of crimson a killer sees before he draws his final breath. I am vengeance. I am the night. I am the Red Death.
Jenna West comes back for the first time since Season 6, now a toddler.
Roy Bivolo comes back from an even longer bus trip, having last been seen all the way back in Season 1.
Call-Back: Once again, an evil speedster wishes to use Barry's speed to help them return home, and when Barry thwarts these plans, they fly into a rage and seek the ultimate revenge.
Entertainingly Wrong: Red Death herself is entertained at Barry wrongly assuming that she is the Negative Speed Force's avatar.
Exact Words: Ryan says that using super speed she could stop crime before people think about committing them. Iris eventually catches on that it's less ethical and more obsessive than it sounds.
Hypocrite: Despite claiming that there is no such thing as a reformed criminal and berating her version of the Flash for working with the Rogues, Red Death herself is also working with criminals.
I Choose to Stay: At the end of the episode, Joe decides to stay in Central City after all.
I Have Your Wife: The Red Death captures Iris to force Barry to power her cosmic treadmill.
Internal Reveal: Barry, Iris, and the rest of the team learn who Red Death really is.
Knight Templar: Red Death seems to genuinely believe that her Flash is the bad guy and that imprisoning people who haven't done anything yet is justified.
The Multiverse: Maybe. Following the Crisis it's made clear that there is actually a new multiverse, but only the audience is aware of this—not the heroes, who believe that they are alone on Earth-Prime. Red Death claims she's from an "alternate timeline" (which could bring to mind something like Savitar), but everything about it seems to suggest a parallel Earth similar to what the multiverse was pre-Crisis.
My Greatest Second Chance: The reason Joe wants to move out of Central City is because he wants to give Jenna the normal childhood that Barry, Iris, and Wally never had.
Red Death is full of classic Batman references, such as calling criminals a "superstitious, cowardly lot." She also uses the 1989 Batman movie's phrase "You want to get nuts? Come on. Let's get nuts."
Bivolo's eyes glow yellow when he induces fear. Yellow is the color associated with that emotion according to the Green Lantern mythos.
When Red Death mocks Barry's theory that she's the avatar of the Negative Speed Force, she says she's not "some kind of herald for the dark cosmos". Comicbook Red Death was a herald for the Dark Multiverse.
No One Gets Left Behind: Subverted. Barry wanted to get Mark, but with the Red Death recharging, the Rogues forcibly drag him and teleport him back. He's not happy about that, especially about the fact that they don't seem bothered by abandoning Mark.
Obviously Evil: When Red Death first shows up in the plaza without even doing anything, the people run for their lives, as they can instantly tell this person is bad news.
Pet the Dog: Mark keeps Mercer from unmasking Barry by informing him that the cowl is rigged to shock someone who tries to remove it.
Pint-Sized Powerhouse: It's made clear in this episode that Red Death's height is comparable to Javicia Leslie's (even if there may be a stuntperson in the suit for certain scenes), which is 5'3", far shorter than past iterations of the "evil, menacing speedster" trope frequently seen throughout the series. Of course, she is still portrayed as no less dangerous or menacing.
Redemption Equals Death: Mark turns against Red Death and frees Barry from the treadmill, but is seemingly killed holding off her Rogues while he escapes.
The Reveal: The Red Death is not the new Negative Speed Force Avatar that will replace Thawne, but actually an alternate timeline version of Ryan Wilder who was adopted by the Wayne family and became a vigilante after their murder. But she later became so obsessed with crime-fighting that she began copying her enemies' weapons and powers, including the Flash's. She used artificial speed to became a ruthless vigilante who killed people who hadn't committed crimes yet. The Flash led Rogues of his own to stop her, ending with her killing Iris when she tried to get her to stand down, then hid in the Speed Force from him only to wind up in the main timeline. Now she seeks to build a cosmic treadmill to get back to her timeline, and take the prime Iris with her to use as leverage against her Flash.
Revenge: Roy Bivolo is still angry at Barry for being locked up in the Pipeline for months.
Spot the Imposter: Iris quickly realizes that Red Death is not the Ryan she knows. Her first clue is how even the fastest Batmobile couldn't get from Gotham to Central City in a day as well as how Ryan acts like she's been in Iris' home scores of times before.
Surprisingly Realistic Outcome: The Rogues helping Barry don't really care that they abandoned Mark to his death, saying that he betrayed them.
Took a Level in Badass: Roy Bivolo's powers have evolved to the scale of Carrie Bates', where he can make people feel other emotions besides anger. He uses it to make Barry feel fear.
Uncertain Doom: We last see Mark holding off the Rogues and Red Death as the rest of the heroes escape, but we don't actually see what happens next.
The Un-Reveal: It's still not made clear what happened to the prime timeline's version of Ryan Wilder after Red Death showed up.
Villainous Breakdown: With the cosmic treadmill destroyed and her plans to return home (literally) up in flames, the Red Death decides to get revenge on the prime Flash as her way of "taking the war to him this time".
