The Satsui no Hado made its debut during the first World Warrior Tournament, where Ryu unknowingly awakened something 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 darkness inside of Ryu until the events of Street Fighter Alpha 3.
The Satsui no Hado is a form of ki that is rooted in the darker aspects of the 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 Hado, a person must be so consumed with the desire for winning and perfection, be close to being killed, or else possess such intense rage and hatred, that they are willing to kill. As such, only people who push themselves to be the best at any cost can take full advantage of it, however, the untrained mind can be driven into insanity if they are not ready for it. Learning the Satsui no Hado 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 ChangesEdit
For the people who do succeed in controlling it, they become extremely powerful, but at the cost of losing their humanity and compassion; they develop a cold, negative outlook on life and a nihilistic detachment from themselves, as well as other people, and develop an abessive, dark, murderous (and even suicidal), desire to fight.
The user also gains the ability to sense, and potentially influence, the emergence of the Satsui no Hado in others. Once a user has tapped into the Satsui no Hado within them, it can be very hard to resist. Within the main series, Goutetsu is the only person known to date that was able to utilize its power without completely giving in to its murderous and darkest urges.
When under the Satsui no Hado's influence, a person's eyes will glow white and red and gain sharp fangs. 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 cannot be released from its hold and will transform into a demonic creature, likely one similar to Oni.
Goutetsu himself was once seen meditating while tapping into the Satsui no Hado, 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 sparingly, which is why he kept his human facial appearance and did not demonstrate signs of corruption. This was also how both of his students, Gouken and Akuma, were introduced to its power.
Goutetsu must have taught Satsui no Hado to his pupils if he also taught them the Shun Goku Satsu, however Ryu (only while under the influence of Satsui no Hado) can use this technique despite the fact that Gouken never taught it to him. Thus implying that the technique is an inherent quality of the dark power.
Once Akuma fully gave in to Satsui no Hado, he lost all of his compassion and humanity as a human, often being described as a demon. and became a person who's only desire to only seek combat, power, and search for a fight that will cost him his life.
Goutetsu was actually pleased and somewhat impressed with his student Akuma for tapping into Satsui no Hado during a life or death battle for control of their school - or so he thought. Akuma wanted to tap into the Satsui no Hado to become far more powerful, and wanted to use it for killing people who he found worthy of killing, rather than using it for enlightenment and good like Goutetsu. Goutetsu may have been unaware that his student had completely lost his compassion for others.
However, Gouken, taught a alternative derivation of the style by removing the dark power, detested the Satsui no Hado, as he believed it encouraged killing after seeing how Akuma used it to murder Goutetsu. He condemned his brother Akuma for corrupting his Chikara no Hado (力の波動 Chikara no Hadou?, "Surge of Power") and vowing to never teach it to anyone.
Although Akuma may have tamed the Satsui no Hado, he could not stop the side effects of this dark force, although it is highly likely he had no desire to. His understanding of the Satsui no Hado is so great that he can embrace it to a much 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 is the result of Akuma fully 'merging' with the Satsui no Hado. As explained earlier, his appearance has changed to that of a larger, demonic being, with a supernatural power beyond that of Shin Akuma.
During Street Fighter Alpha 2, Ryu finally realized that the Satsui no Hado was the culprit behind his aforementioned victory over Sagat. Ryu vowed to reject the Satsui no Hado, because he knew that he could not control this dark force, and thus could not risk killing anyone he fought or losing his compassion and humanity.
Although Ryu never shows any signs of the Satsui no Hado's influence after the events of the Street Fighter IV series, several games present a hypothetical version of Ryu who has embraced the power, willingly or not, becoming Evil Ryu. Furthermore, other media often depicts him temporarily succumbing to the Satsui no Hado 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 an appearance similar to that of Akuma, possibly to show how he would transform if the seal was broken.
In the events of UDON's Street Fighter Legends: Sakura miniseries, Dan falls to the dark power while fighting Sakura. However, Dan's attempt to perform the Shun Goku Satsu on her fails by resulted in him tripping over her backpack and rendering himself unconscious by Sakura's attack.
Dan has a different kanji appearing on his back, showing the kanji (父 chichi?, "father"). It is possible Dan's power stems from the feelings of hatred that he harbours towards Sagat for murdering his father.
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 powers, and her personality remains mostly unchanged. This also makes Sakura the only female to have used the Satsui no Hado at any point.
In Udon's Super Street Fighter comics, Sakura actually falls to the Satsui no Hado while fighting Akuma and Ryu. 
Known victims of the Satsui no HadoEdit
- Gen - Defeated and seemingly murdered by Akuma's Raging Demon in Street Fighter Alpha 3, but survived by emptying his soul.
- Gill - Though disputed whether canon or not, he was defeated and seemingly murdered by Akuma's Raging Demon in Street Fighter III, but recovered via resurrection.
- Gouken - Defeated and seemingly murdered by Akuma's Raging Demon, but survived by emptying his soul using the Power of Nothingness.
- M. Bison - Though disputed whether canon or not, he was defeated and seemingly murdered by Akuma's Raging Demon in Super Street Fighter II Turbo, but survived the attack by putting by a part of his Psycho Power in another body.
- Shun - Murdered by Evil Ryu during the events of Street Fighter Alpha: The Animation (non-canon, alternate continuity).
- While Kairi from the non-canon Street Fighter EX games is not a user of the Satsui no Hado, the power he wields works in a similar fashion.
- The being Garuda, is also said to be linked to said power in the EX games.
- In the iOS game Street Fighter IV Volt, the Satsui no Hado appears as a skill in Wandering Warrior. However, one would need the title "Messatsu!" to get this skill. When this skill activates, the player's avatar endures the hit that would have cost the avatar the match with 1 HP remaining, and then, it activates. While this skill is active, the damage the player's avatar scores on the opponent's avatar is increased.
- In Capcom vs. SNK 2, Rugal Bernstein, after defeating Akuma, mixed the Satsui no Hado with his own Orochi powers and becomes God Rugal (sometimes referred to as Ultimate Rugal).