Evil Ryu

In the Street Fighter Alpha series, there is a selectable version of Ryu called Evil Ryu known in Japan as Satsui no Hadou ni Mezameta Ryū ("Ryu who has Awakened to the Surge of Killing Intent"). He was originally introduced in a Street Fighter Zero manga series authored by Masahiko Nakahira and later adapted in the Street Fighter canon by Capcom. He is the version of Ryu should he succumb to Satsui no Hadou, and depicts Ryu's struggle to resist the urge. Gameplay-wise, Evil Ryu has more powerful attacks than the original, strikes faster and possesses some of the attacks of Akuma (such as the Shun Goku Satsu). Like Akuma, Evil Ryu has a signature symbol that appears when he performs the Shun Goku Satsu. Evil Ryu has a different symbol: in Alpha 3 and Capcom vs. SNK 2, his symbol was Metsu, for "Destruction" (滅), though it only appears in his ending, not during gameplay.

History
The only time Evil Ryu actually appears in Street Fighter canon is at the end of the first World Warrior tournament. According to the Street Fighter Alpha series, Sagat, the defending champion, crushed Ryu, and then extended his hand to help Ryu up after thinking that he had won. Ryu was so consumed with the desire to win that he gave into the Satsui no Hadou ("Evil Intent" in the American version) and executed a Metsu Shoryuken, scarring Sagat's chest and his pride.

Since then, Evil Ryu has occasionally appeared in various Capcom games featuring Ryu. Nevertheless, Evil Ryu does not exist as a separate character and is only an alternate, "what if" version of Ryu. Despite the brief brush and/or temptation from the Satsui no Hadou, canonically, Ryu rejected the Satsui no Hadou, and has made mass innovations to Gouken's style which helped him bring the fighting style up to par with Evil Ryu by Street Fighter III. However, there is evidence which can be interpreted otherwise in the home versions of Street Fighter Alpha 3; if the player matches normal Ryu against Evil Ryu, they have a special pre-match introduction that involves normal Ryu attacking first with a rushing elbow, and then Evil Ryu counterattacking, suggesting either a fierce rivarly between the normal Ryu and what supposedly appears to be an "evil clone" of him or that the battle is taking place inside Ryu's mind, representing his struggle with the Satsui no Hadou.

Satsui no Hadou
To tap into Satsui no Hadou, a fighter must be so consumed with the desire to win that they are willing to kill. As a result, only people who push themselves to be the best at all costs can take advantage of it. However, the untrained mind can be driven to a temporary insanity if they have not prepared for it. For those who do succeed in controlling it, they develop an almost cold, clinical detachment from themselves. After Akuma tamed the Satsui no Hadou, he lost all of his humanity and became a fighter who only seeks to become the strongest in the world. As such, Satsui no Hadou makes a human unbelievably powerful but at the cost of losing one's humanity. Akuma's understanding of Satsui no Hadou is so great, he can seal himself off from it, giving lesser opponents a fighting chance. Learning Satsui no Hadou is the first step in learning the most fatal technique known to all of their martial art, Shun Goku Satsu.

Goutetsu himself was once seen meditating while tapping into Satsui no Hadou, so he probably encouraged, or at least accepted, it. This was also how both his students Gouken and Akuma were introduced to the power. Goutetsu would also have had to teach Satsui no Hadou to his pupils if he did teach them the Shun Goku Satsu. Finally, he was actually pleased with his student Akuma for tapping into Satsui no Hadou during a life or death battle for control of their school. Gouken, a student of Goutetsu who taught a toned-down derivation of his style, detested Satsui no Hadou since he believed it encouraged killing. He condemned his brother Akuma for corrupting his Chikara no Hadou (Surge of Power).

Originally, Ryu scarred Sagat with a single Shoryuken. It was later retconned that Ryu lost control of himself by rising up fist first, upsetting Sagat (and nearly killing him) with a Metsu Shoryuken. His attack took on the properties of Goutetsu's original art despite having never been taught it by his master Gouken. Although Sagat wanted revenge for his scarred chest, he never knew that the scar came from the Satsui no Hadou, a dark power that consumed Ryu. During Street Fighter Alpha 2, Ryu finally realized that the Satsui no Hadou was the culprit behind his cheap victory over Sagat. Knowing about this dark intent, Ryu has vowed to reject the Satsui no Hadou. It was not until the end of Street Fighter IV that Gouken was able to seal it away using the "Power of Nothingness".