The Flash's Rogues are considered unambitious by villain standards, but they regularly take on the Flash and get away with it. He can't even keep them in prison.
They have shown themselves to be extremely powerful as well. Heat Wave has used flamethrower pistols to burn the Flash, a speedster with resistance to heat and friction. Heat Wave even managed to burn a Black Lantern. Normally you need two different colored Lanterns to defeat a single Black Lantern. Captain Cold is the only person capable of freezing anything to Absolute Zero, a feat even Victor Frieze can't accomplish. Mirror Master can do anything with any mirror: shoot lasers, travel through them, even see through them. He can even turn someone into a mirror. Weather Wizard can blast you with lightning, call down a tornado, or just freeze you out with a blizzard. Doctor Alchemy can transform all sorts of stuff, like human blood to formaldehyde when it's still in a living body. Captain Boomerang, a fat, cowardly Australian nationalist who can throw trick boomerangs really well has the highest body count of any of the Rogues I've named here. He also banged the mom of one Flash.
Together, the group has managed to kill one Flash (Bart Allen) whent ricked into it, and then killed the Reverse Flash who did the tricking. No killing speedsters, women, or children. That's in their rules.
As for backstory...whew...they got some dark ones. Abuse, pyromania, killing members of their own families on purpose or by accident, working as hired killer before getting gadgets and a costume.
They also got balls. When Darkest Night happened and dead heroes and villains were returned as Black Lanterns seeking to tear the hearts out of their former friends, family, and foes while causing as much emotional turmoil as possible (they see via emotion and used it to power their Black Lantern Battery).
Their response to knowing that their own dead friends(and dead baby sister in Captain Cold's case) were rising up to tear out their hearts?
Hunt them down first, of course.