Rais - The Leader (Dying Light)
Suleiman (Rais, meaning 'Leader' or 'Chief') is ruthless, sadistic and violent. He readily enjoys using aggressive tactics to get what he wants, and doesn't hesitate to use lethal force with or without reason, based on his mood. He is a brilliant and manipulative tactician capable of bending the weaker-minded to his will. It isn't known if his violent nature comes from a need for complete dominance, natural inclination or a break in his mental sanity.
Rais isn't afraid of blood, and is known for punishing those around him for displeasing him. He kills, dismembers or injures friend and foe alike, for even the most trivial reasons. (See video below)
Deadpool w/ Madcap - The humorously unhinged duo (Marvel Universe)
As we all know, Deadpool is a few crayon's short of a full box; but with Madcap he has met his match. Discovering during a battle that they both have the same regenerative abilities, Deadpool is thrilled by the challenge and instigates further fighting. Madcap has the ability to drive people crazy for a period of time, he uses the ability on Deadpool but it has the opposite effect; it makes him sane.
Encountering Deadpool in the middle of a fight with Thor and Daredevil, Thor accidentally reduces both Madcap and Deadpool to ash. Deadpool regenerates, seeming normal at first, until he hears Madcap in his head; realizing that they had regenerated as one (Similarly as Firestorm in DC Universe). Madcap temporarily becomes an addition to the many voices inside Deadpool's head; with the addition of being able to assert himself and control Deadpool. Madcap no longer wanting to be just a voice in Deadpool's insane mind, takes control of Deadpool's body and uses his mental powers of manipulation to force Thor and Cage to rip him into two. Deadpool and Madcap then regenerate individually from the two halves.
These two make a perfect duo; a combination of Firestorm, and Captain Cold with Heatwave, they compliment each other. Apart, they are insane and morally flexible anti-heroes, but together... they are magic.
Krieg - The Bloodthirsty Barbarian ( Borderlands 2 )
"Whatever Krieg went through to make him the blood-drinking, meat-loving killer that he is, it is noted he wasn't always like this. Krieg's psyche has a remnant of who he used to be--a literal inner voice which forces Krieg to only kill the deserving, and help others. The voice often remarks that he remembers helping people, getting paid with loot, and other Vault-hunter-esque pursuits. It's also informed Krieg that should Krieg ever kill an innocent person, the inner voice will take over and commit suicide to kill them both as punishment." http://borderlands.wikia.com/wiki/Krieg
GLaDOS - The Sinister and Unfeeling AI ( Portal 2 )
Having apparently killed other testers before her fail safes were installed, GLaDOS is the AI that leads players through Portal 2. She is witty, snarky, narcissistic, sinister and hugely passive-aggressive. Spending most of her time trying to convince you how much of a horrible person you are and how nobody likes you, it seems GLaDOS was installed not only to guide you but test your self-esteem and capacity for verbal and mental abuse.
GLaDOS could probably single handedly convince someone to kill themselves if she chose, since the fail-safes mean she can no longer kill anyone herself. Given that she can also be implanted in objects (and assuming people) makes her a particularly deadly addition to the team