Satsui no Hado

The Satsui no Hadou (殺意の波動, Satsui no Hadou, lit. "Surge of Murderous Intent"), also known as the Dark Hadou (ダーク ハドウ), is a evil, demonic energy that is most known for its use by Akuma, as well as Ryu's struggle against it.

Appearances
The Satsui no Hadou made its debut during the first World Warrior Tournament, where Ryu was possessed by and awakened this ki inside of him to defeat Sagat in a drastic effort.

Originally, Ryu scarred Sagat with a single Shoryuken. It was later retconned in Street Fighter Alpha that Ryu lost control of himself and his rage by rising up fist first, scarring Sagat with a Metsu Shoryuken instead. 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 something that had awakened the evil inside of Ryu.

Description
Satsui no Hadou is a form of ki that is rooted in the darker aspects of natural human instinct (including the desire to survive, trample, and impose over opposition), and, on a metaphysical level, the natural occurrence and manifestation of loss, decline, and destruction.

To tap into the Satsui no Hadou, a person must be so consumed with the desire to win, or else possess such intense rage, that they are willing to kill. As such, only those who push themselves to be the best at all costs can take full advantage of it. Learning Satsui no Hadou is the first step in learning the most fatal technique known to all of their martial arts - the Shun Goku Satsu, known also as the Raging Demon.

Physical and Mental Changes
For the people who do succeed in controlling it, they become unbelievably strong, but at the cost of one's humanity and compassion; they develop a cold, cynical detachment from themselves, as well as others, and develop a lust to fight to the death. Such people also gain the power to sense and influence, the emergence of Satsui no Hadou in others. Once a person has tap into the Satsui no Hadou within them, it can be very hard to resist. Goutetsu is the only one to date that was able to utilize its power without completely giving in to murderous and dark urges.

When under the Satsui no Hadou's influence, a person's eyes willl glow white and red. If one embraces it to a lesser degree, their eyes will glow blue. If one has used it to their fullest extent for too long, they will transform into a demonic being, likely one similar to Oni.

Goutetsu
Goutetsu himself was once seen meditating while tapping into the Satsui no Hadou, so he probably encouraged or at least accepted it, but he did not lose his humanity like Akuma. This is likely because he only used it to a degree, which is why he kept his human facial appearance and did not demonstrate signs of corruption. This was also how both his students Gouken and Akuma were introduced to its power.

Goutetsu would also have had to teach Satsui no Hadou to his pupils if he did teach them the Shun Goku Satsu, but Ryu (only in his evil state) can also use this move despite the fact that Gouken never did teach him the move, meaning that the move is an inherent quality of the dark power.

Akuma
After Akuma fully gave in to the Satsui no Hadou, he lost all of his humanity, as well as his compassion as a person, to the point of becoming a person who only seeks a fight that will cost him his life towards another warrior and a fighter that only wants more power and took a demonic appearance to match his darkened soul.

Goutetsu was actually pleased and somewhat impressed with his student Akuma for tapping into Satsui no Hadou during a life or death battle for control of their school - or so he thought. Akuma wanted to tap into Satsui no Hadou to become far more powerful, and wanted to use it for fighting to the death against warriors who he found worthy of killing, instead of using it for enlightenment and good. Goutetsu may have been unaware that his student had lost all of his humanity and his compassion as a person.

However, Gouken, a student of Goutetsu who taught a toned-down derivation of his style by removing the dark power, detested the Satsui no Hadou, he believed it encouraged destroying after seeing how Akuma used it to murder Goutetsu. He condemned his brother Akuma for corrupting his Chikara no Hadou (Surge of Power) and vowing to never teach it to anyone.

Akuma may have tamed the Satsui no Hadou, but he could not stop the side effects of this unlimited power, although it is highly likely he had no desire to. His understanding of Satsui no Hadou is so great, that he can seal himself off from it or embrace it to a lower degree, giving lesser opponents a fighting chance. However, he still cannot return to his human self, as he has used it to his fullest extent for too long.

Oni
Oni is the result of Akuma finally becoming one with the Satsui no Hadou. As explained earlier, his appearance has changed to that of a larger, demonic being, with a supernatural power beyond that of Shin Akuma.

Evil Ryu
During Street Fighter Alpha 2, Ryu finally realized that the Satsui no Hadou was the culprit behind his aforementioned victory over Sagat. Ryu vowed to reject the Satsui no Hadou, because he knew that he cannot control it, and does not want to kill any of the warriors he fights. Furthermore, even if he learned how to control it, he would still lose his humanity like Akuma, as Evil Ryu's personality shows.

Although Ryu never shows any signs of the Satsui no Hadou's influence after the events of the Street Fighter Alpha series, several games present a hypothetical version of Ryu who has embraced the art, willingly or not, becoming Evil Ryu. Furthermore, other media often depicts him temporarily succumbing to the Satsui no Hadou and assaulting an opponent before returning to normal.

It was not until the end of Street Fighter IV that Gouken was able to seal it away using the "Power of Nothingness" to help him control it like Goutetsu; however, it is unknown how long the seal will be able to hold his dark power. A demonic version of Evil Ryu appears in the Arcade Edition of Super Street Fighter IV with a redesign and a more powerful boss form, possibly to show how he would change if the seal was broken.

In addition, Ryu and Ken's ending in the crossover Street Fighter X Tekken suggest that Pandora's power is trying to merge with the Satsui No Hadou within him.

Dan Hibiki
In the events of the Sakura Udon miniseries, Dan Hibiki seemed to succumb to the evil power while fighting  Sakura. However, Dan's attempt to perform the Shun Goku Satsu resulted in him tripping over and rendering himself unconscious, in an example of comedy relief.

Dan has a different Kanji appearing on his back, showing the symbol "父" (father in Japanese). It is possible Dan's power stems from the feelings of revenge he harbours towards Sagat for murdering his father.

Sakura Kasugano
In Marvel Super Heroes vs. Street Fighter, there is a secret character known as Dark Sakura (also called Sunburnt Sakura), a version of Sakura who utilizes a mixed moveset of Akuma and Evil Ryu. Unlike Ryu and Akuma, Dark Sakura is able to control her dark powers and her personality remains mostly unchanged. This also makes Sakura the only female to have used the Satsui no Hadou at any point.

Known victims of the Satsui no Hadou

 * Goutetsu - Killed by Akuma in an honorable death match.
 * Sagat - Scarred and defeated by Ryu's Metsu Shoryuken.
 * Shun - Falls victim to it and dies during the events of Street Fighter Alpha: The Animation (non-canon).
 * M. Bison - Defeated and seemingly killed by Akuma's Shun Goku Satsu in Street Fighter ll.
 * Gill - Defeated and seemingly killed by Akuma's Shun Goku Satsu in Street Fighter llI, but recovered via resurrection.

Trivia

 * While Kairi of the non-canon Street Fighter EX series is not a user of the Satsui no Hadou, the power he wields works in a similar fashion.
 * In addition to Gouken, the Satsui no Hadou is opposed by Ken, Ingrid, Juri, Guy, Sagat and Rose.