Satsui no Hadou
The Satsui no Hadou is the known darker form of energy used in many forms of Japanese Martial Arts (Ansatsuken in particular). This form of energy is usually fed from one's own desire to live or to win in some kind of competition or another form thereof. Usually when one is completely desperate need to succeed or to survive, and that one must be willing to kill in order to achieve their goal. The Satsui no Hadou gives the initiate incredible power but at the cost of one's humanity and compassion.
Gouken and Akuma's master, it is said that he had used the Satsui no Hadou. But for unknown reasons (possibly his training) he didn't succumb to the Hadou's corrupting side-effects. But this very Hadou is the reason why Gouken decided to leave Goutetsu, but Akuma decided to stay and learn about this Hadou. Goutetsu had a fight to the death against Akuma who killed him eventually with the Raging Demon.
Akuma wanted to learn more about this energy that Goutetsu had harnessed. But Akuma killed his master with the Raging Demon, forcing Akuma to learn more about this Hadou on his own. Unfortunately, Akuma didn't know how to not succumb to the corrupting effects of the Hadou, he loses his humanity and only seeks strong opponents in order to train more with this energy. So, Akuma has learned to train alongside the Hadou to perfect his skills. But despite the Hadou. Akuma has his own codes to follow by and will rarely (if at all) murder innocents and only kill a worthy opponent in combat. After Gouken teaches the Power of Mu (nothingness) to Ryu, which will suppress the Satsui no Hadou within him. He fights Gouken to see who is really worthy of Ryu's being. The outcome of the fight is unknown. But Akuma would never get the death match he seeks against Ryu.
Ryu has succumbed to the Satsui no Hadou only a handful of times during his journey. The first time is when he was defeated by Sagat in the final battle in the World Warrior tournament. Consumed by his desire to win and not taking his loss very well, he revived with the help of the Satsui no Hadou and performed the Messatsu Shoryuken on Sagat, thus defeating (and scarring) the Muay Thai master. But as Ryu beats Akuma on a deserted island off the coast of Japan, he learns of the Satsui no Hadou. He dreams of fighting his evil self but awakens before Ryu's evil self were to kill him. He succumbs to the Satsui no Hadou after he was defeated by M.Bison who infuses Ryu with the Psycho Power (which Bison hopes will give him the body he longed for) he was defeated by Sagat who in turn encourages him to overcome the Hadou inside Ryu. After Ryu defeats Bison, he returns to Japan, only to begin another journey, one that will allow Ryu to suppress the Satsui no Hadou within him and instead of either letting it control him or be one with it like Akuma. It wouldn't be until after the World Warrior tournament sponsored by S.I.N. where Ryu has met his (presumed dead) master: Gouken. After a sparring session, Ryu tells Gouken about the Satsui no Hadou inside of him. Understanding of his situation and always willing to help Ryu, Gouken decides to teach Ryu about the Power of Mu (nothingness) to gradually suppress the Satsui no Hadou within Ryu. As Ryu continues to train the power of Nothingness, he is nearly consumed by the Satsui no Hadou twice (once in a sparring session with Gouken and another when he lost to Necalli). But as he fights Necalli again, he uses the power of Mu to eliminate Necalli. Ryu has done this again when he fights M. Bison as he uses a Mu powered Hadouken which causes Bison's body to shatter.
 Non-Canon Users
 Ken Masters
In SNK VS. Capcom: SVC Chaos, you can pick an evil version of Ken Masters known as Violent Ken. Whether or not if this is Ken when he succumbs to the Satsui no Hadou is unknown.
in Marvel VS. Capcom 2, if you choose Sakura, she has a super combo move which turns her into a somewhat evil version of herself known as either Dark Sakura or Sunburned Sakura. She has the same moves as Akuma. But the character herself isn't very well developed storywise. So it's unknown if this is Sakura with the Satsui no Hadou.