Satsui no Hado

The Satsui no Hadou (殺意の波動, Satsui no Hadou, literally "Surge of Murderous Intent"), also known as the Dark Hadou (ダーク ハドウ, Daaku Hadou, Dark Surge), is a type of chaos energy and a demonic force 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 energy 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 the Satsui no Hadou that had awakened something inside of Ryu.

Description
Satsui no Hadou is a form of energy that emanates greatly from oneself through the darker aspects of natural human instinct and the desire to survive, to 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 fighter must be so consumed with the desire to win or have such an intense rage, that they are willing to kill. Only people who push themselves to be the best at all costs, can take full advantage of it. However, the untrained mind can be driven to a temporary insanity.

For those who do succeed in controlling it, they develop a cold, cynical detachment from themselves, as well as others. As such, Satsui no Hadou makes a person unbelievably strong, but at the cost of one's humanity and compassion.

Another side-effect of this power is that when a warrior embraces the Satsui no Hadou to their fullest extent - even when they can not control it - they will develop a lust to fight to the death, their eyes will turn white or red, and their beliefs in fighting become very dark and cold-blooded. The warrior's personality will also change and become very violent and dark. However, as shown in the case of Goutetsu, if one embraces it to a small degree, their eye's pupils will become blue, and they will have no willingness to destroy anything or anyone.

Once a warrior has awoken the Satsui no Hadou within them, it can be very hard to resist. Learning Satsui no Hadou is the first step in learning the most fatal technique known to all of their martial arts - Shun Goku Satsu. If one has used it to their fullest extent for too long throughout years of their life, 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 any of the side effects. 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, 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 fighter who only seeks a fight that will cost him his life towards another warrior and a fighter the 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 most of his humanity and compassion.

However, Gouken, a student of Goutetsu who taught a toned-down derivation of his style by removing the murderous intent, truly detested the Satsui no Hadou, since he believed it encouraged killing after seeing how Akuma used it to kill 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 power beyond that of Shin Akuma.

Ryu/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 uses the Satsui no Hadou after the events of Street Fighter Alpha 2, 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. Evil Ryu reappears in the Arcade Edition of Super Street Fighter IV, and possesses a more powerful boss form, possibly to show how he would change if the sealing had failed.

Dan Hibiki
In the events of the Sakura Udon miniseries, Dan Hibiki seemed to succumb to the dark power when he'd taken damage from Sakura. Dan had 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.

However, Dan's attempt to perform the Shun Goku Satsu resulted in him tripping over and rendering himself unconscious; thus, this example is most likely comedy relief as opposed to being actual canon, since Dan has yet to be shown using the Satsui no Hadou in the storyline of the games OR in any other instance outside of this one.

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 with Ryu, who cannot control his dark powers, and Akuma, who is unable to show any compassion or humanity, Dark Sakura is able to control her dark powers and keep her humanity and compassion. However, as this is simply a secret character in a crossover game, this version of Sakura is non-canon.

Known victims

 * 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.

Trivia

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