Name: Satsui no Hado (殺意波動, Satsui no Hadou, literally "Surge of Murderous Intent")
Debut: Street Fighter Alpha
Satsui no Hado is a dark force which Ryu was possessed by to defeat Sagat in a drastic effort.
To tap into Satsui no Hado, 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, cynical detachment from themselves. After Akuma tamed the Satsui no Hado, he lost all of his humanity and became a fighter who only seeks to become the strongest in the world. As such, Satsui no Hado 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. Ryu doesn't want to have The Satsui no hadou inside of him because he knows that he can't control it, even if he learns how to control it, he would still loose his humanity like Akuma. 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 Hado, so he probably encouraged or at least accepted it (but he did not lose his humanity like Akuma). This was also how both his students Gouken and Akuma were introduced to the power. Goutetsu would also have had to teach Satsui no Hado 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 Hado 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 Hado, a dark power that consumed Ryu. During Street Fighter Alpha 2, Ryu finally realized that the Satsui no Hado was the culprit behind his cheap victory over Sagat. Knowing about this dark intent, Ryu has vowed to reject the Satsui no Hado. It was not until the end of Street Fighter IV that Gouken was able to seal it away using the "Power of Nothingness".