As Barry dies from the Anti-Monitor's anti-cannon in Crisis on Infinite Earths, his body wastes away and his face becomes more and more skeletal until it disappears from existence. See the image above.
Black Flash. He is a skeletal wraith monster dressed in a black Flash uniform. In other versions he appears as a Bizarro version of the good guy. Creepy, but okay. He's fast, just like him. Tough, but expected. Unfortunately this doesn't get that far when you know that he's out for your soul if you're a speedster, and you're destined to go. Meet him, and goodbye. He is the Grim Reaper for speedsters, and no matter how fast they run, they will never escape him (except the third Flash).
In order stories he's even creepier. For example, when you realize that the Ame-Comi version (a universe where all DC characters are adorable anime girls) is actually very pretty, but still a zombie.
There's a certain Fridge Horror at play as well, when you really look at his character design. He's a wraith wearing a black version of the Flash suit, yes, but specifically he's wearing Barry Alllen's suit. Then look up at how Barry died, being aged into a skeleton as he ran into nothingness, and then remember that when Barry's revived, he's briefly transformed into the Black Flash himself. Essentially, it was very likely that Wally was being chased by the possessed corpse of his beloved uncle the entire time.
Murmur, another rogue of The Flash, has a healthy bit of this behind him. A serial killer, his modus operandi revolves around his hatred of the sound of speech - he cuts out peoples' tongues, created a quick acting virus that essentially turns your lungs and throat into jelly, and cut out his own tongue and stitched his mouth shut to stop himself from talking.
The Reverse-Flash aka Eobard Thawne. Generally, you get what you'd expect from an Evil Counterpart. Until he decides he wants the real Flash's life, and tries to replace the Flash in his marriage. Which includes living with his wife. When he finds out she doesn't love him (and in fact is repulsed by him) he deals with rejection by liquidating her brain with his finger.
If he can't have that, then ruining Barry's childhood will suffice, from actions as petty to making him miss a thrown baseball to erasing his childhood friends from existence so he grows up lonely and sad. It's an in-story retcon, which, in a way, makes it all the more worse. Plus unlike most speedsters, Eobard Thawne doesn't need the Cosmic Treadmill to time travel. He can run to whatever time period he likes and spread as much terror as he pleases between moments at a time.
Or how about the fact that he already has a family of his own anyway?◊ One can only be grateful that his last remaining heirnote and yes, it is in the sense of blood descendant, i.e. great-great-great-grandson, or however many generations 500 years covers - the superhero Impulse - is about as purehearted as a teenager can be. And for that, poor Bart is targeted for death by virtually the entire Thawne bloodline.
Savitar's death. He is touched by Barry after Thawne interferes with the Speed Force, causing him to age and rot into dust.
The Rogues, or rather the demons occupying their bodies, after Underworld Unleashed. Instead of the colourful bank-robbers, they were turned into sadistic killers who dragged Jay Garrick into hell to torture him, while trapping everyone in Keystone City inside the city borders so they could kill them all.
Note that, while this was happening, the Rogues' souls were trapped in hell. The experience was so bad that Weather Wizard made a Heel–Face Turn to avoid ever going back, while the others took time off to recover.
Hunter Zolomon as Zoom adopts this trope. He murders innocent people close to superheroes, will not hesitate to kill anybody who gets in his way, and he thinks that his actions are improving other heroes. He truly thought that he was helping Wally when he tried to kill his pregnant wife. The worst part? He's even faster than the Flash, and can punch Wonder Woman across entire continents.
Wally can be terrifying if he’s sufficiently pissed off. After Inertia murdered Bart, Wally nearly turned the sociopathic little brat into a bloody smear on the ground before instead deciding to steal his Speed Force, turning Inertia into a statue that would take a hundred years to blink. And for the cherry on top, Wally made a point of putting him on display at the Flash Museum, specifically in the section dedicated to Bart.
New 52
Daniel West as the Reverse Flash. He doesn't wear a costume. Metal from a Speed Force-powered monorail melted onto his skin and what skin remains uncovered is severely burnt.
Rebirth
Bloodwork. He's nothing but a human-shaped mass of blood and blood vessels who can control blood, use it to change his shape into something monstrous, and coat all of Central City in blood.
, and goodbye. He is the Grim Reaper for speedsters, and no matter how fast they run, they will never escape him (except the third Flash).
