Assassins, Freelancers, Mercenaries, and other Hired Guns:
There are always those who are willing to commit all sorts of violence for payment. This is but a short list of the sort of people who can be hired to do something from the messy to the lethal.Key: (P): Mostly Psycho. (S): Mostly Sane.
Bolt (S): Larry Bolatinsky used his skills as a SFX technician to create his Bolt suit, establishing himself as one of the underworld's top assassins. The suit affords him enhanced protection, flight, teleportation and the ability to electrocute his opponent. His code of honour boils down to: Do no freebies outside of self-defence, stay bought if treated fairly or ensure the employer's death if they betray, lie or otherwise deliberately mistreat you as a warning to others.
Cheshire (P): Jade Nguyen is a specialist in hand-to-hand combat and poisons, preferring to coat fake razor-sharp fingernails with poison. Black Canary once called her the second-greatest assassin in the world (with Lady Shiva taking the top slot). She gained history-making notoriety when she stole several nuclear weapons with the aid of Deathstroke in order to blackmail the world. To prove that she wasn't bluffing, she detonated one of the bombs in Qurac, a small Middle-Eastern country. To this day, she is one of the few people on Earth who has a body count in the millions.
Coda (S): Jaime Hernandez specialises in killing superhumans, and in using sonic weaponry. His literally bringing the house down on the cape called Ghosthawk cemented his reputation. Anyone going up against him should know that he doesn't care about a method being "overkill", not when he kills capes for a living.
Deadshot (S): Floyd Lawton is one of Earth's greatest marksmen. He prefers to design and make his own firearms, and he also has a knack for surviving deadly encounters, despite his best efforts to go out in a blaze of glory.
Deathstroke (P): Slade Wilson was a soldier in the U.S. Army, when he received some experimental adrenal serums as part of a super-soldier program. He once managed a near-TPK on the Justice League,
but failed when Green Arrow put an arrow in his right (already blind) eye, triggering a berserker rage. Of all the people listed in this post, his family issues are tied for the most screwed-up along with Cheshire's.
Jade Spider (P): James Jian Kar Fei prefers to use his Jade Spider Style (of which he is the sole surviving practioner, by dint of having killed everyone else to have practiced it) against other martial artists.
Kid Karma (S): Xian Hsui AKA John Soo is a crack shot, whose power of probability manipulation makes him a force to be reckoned with. What gives him an edge is the Wujen ritual he performs every sunset, which ensures that his guns never run out of ammo. He basically has an 'infinite ammo' cheat that works in real life!
Killshot (S): Killshot is believed to be ex-Mossad, which alone would make her formidable. The fact that she has superhuman agility and is also a crack shot makes most people worried when she's after them.
Lady Shiva (S): Sandra Wu-San is one of the few people to leave Ra's Al Ghul's League of Assassins and survive the experience intact. She is considered by most who know her to be the world's greatest living martial artist, which is something that not even the likes of Batman or the second Raven can claim. She has helped train The Question (Vic Sage) and Batman.
Lady Vic (S): Lady Elaine Marsh-Morton is a world-class assassin descended from a long line of soldiers and mercenaries. She works for the money because her aristocratic British family is rich with titles and honours, but poor in cold, hard cash. The commissions she earns pay to keep her ancestral estate from being foreclosed on, and her family dishonoured.
Lobo (P - and how!): This intergalactic bounty-hunter is known throughout the universe as one hardcore son-of-a-bitch! Rumour has it that it's impossible to kill him.
The rumour is true to the point that both Heaven and Hell have barred Lobo from ever entering their realms again. Anyone with Lobo after them had better think of a way to convince him to leave them alone.
Orion the Hunter (P): Jack O'Ryan honed his skills by hunting big and dangerous game around the world. He had mastered the hunt so well, that the only way he could still have a good hunt was to hunt humans and then he moved up to superhumans. He has a major beef with Lady Liberty, who was the first target he ever failed to kill.
Recall (S): Due to his photographic reflexes, he prefers to fight martial artists and weapon-users. The one encounter he has had with Lady Shiva is considered to be one of the most viewed fights on-line of all time.