<reposting this from Capcom-Unity to hopefully spark some discussion here>
// Please keep in mind this stuff is all my interpretation and opinion backed up with explainations and citations where-ever and whenever possible! Also I am always happy to discuss how or why I came to certain conclusions because I love the sort of discussion that delves into canon //
In the interest of some discussion, I figured I'd post up some of my ramblings on one of the more mysterious parts of Street Fighter: The infamous Shun Goku Satsu. As a little heads up, this post won't be short! Best of luck to anyone reading it all and thanks to anyone that gets through to the end and replies! Now, as a fair warning, the Shun Goku Satsu/Instant Hell Murder/Hundred Demon Attack/Raging Demon/wowthishasalotofdifferentnames is a very vaguely detailed technique so quite a bit of it is going to be based on interpretation. I'll attempt to explain my reasoning behind my interpretations as best as possible.
Our actual understanding of the SGS is actually less limited than one might think since we have about 20 years of informational tidbits to draw from. Our best source to start from is this excerpt reproduced in the Plot Guide regarding that very move. I'll break it down and interpret it as best as I can, step by step.
Master Goutetsu spoke of a forbidden technique, "shungokusatsu." With that move, it was said that even one's own life would be also cut off in a moment of unparalleled intensity. Namely with one to have made use of this "satsui no hadou" would be impossible to control, to run wildly until that life's very end, so it was said. Master Goutetsu spoke of a forbidden technique called the "Shun Goku Satsu". A move so unmatched in its intensity that it would even cut off the life of the user. That is to say, the "Satsui No Hadou" (Surge of Killing Intent) used with it would become impossible to control, causing the user to become beserk for the remainder of their days. [Here we have a brief look into the SGS. It is a move so dangerous it even threatens the users own life by way of consuming it. The SGS is fueled by Satsui No Hadou, the killing intent that Goutetsu's fighting style is based on. Unlike the other moves of the style, however, the intensity of the SGS causes that killing intent to go out of control...it consumes the user and makes them, for lack of a better word, crazy for the rest of their life. We got an obvious glimpse of this with Evil Ryu who is mentally consumed by the Satsui No Hadou...unlike Akuma, he's a rampaging madman that kills everything in his path. This is the threat of dipping to deep into Satsui No Hadou, and the SGS was thought to be impossible to use without being consumed.]
Gouki, pronouncing all ancestors "fools" for sealing this "shungokusatsu," selected his path to break that ban on his own. "To become fighters, true warriors of the fist must possess all power that can be utilized. Those who ban techniques cling to life in stinginess, which this warrior of the fist does not!"
Gouki, deciding his forebearers were "fools" for forbidding the "Shun Goku Satsu", decided that he would be the one to lift the ban on it. "To become fighters, warriors of the fist must possess all power they can. Those who would ban a technique are cowards, but this warrior is not!"
[Pretty simple stuff here. Gouki/Akuma, being the total boss that he is decides that he's going to be the one that resurrects the SGS.]
Gouki filled himself with confidence. Having controlled the ancestors' "satsui no hadou", the "shungokusatsu" was utilized. But Gouki's own self changed. Usually when collecting "satsui no hadou", to acquire manipulation of it many years past became a piling heavy burden. "Who exists that is stronger" now became the wish, and this request would be answered with the spraying of blood incited by training. The forbidden move was done. He selected Master Goutetsu as his opponent. As there was about to be a fight to the death, the opponent had no deficiencies.
Gouki was brimming with confidence. Having mastered the "Satsui No Hadou", the Shun Goku Satsu was achieved. But with it, Gouki changed. Over the years, usage of the "Satsui No Hadou" bore down as a heavy burden on the user. "Who is stronger?" became the single question, and the only way to answer it was with the spilling of blood in combat. The forbidden move was unlocked. Master Goutetsu was chosen as the first opponent. Since it was to be a fight to the death, the opponent had to be flawless.
[Here we have Akuma, quite pleased with himself that he has mastered the SGS. However, though he hasn't been consumed by the Satsui No Hadou, he's still been warped by it. All you have to do is look at Akuma to know that he's been strongly inflluenced and changed by the power of the Satsui No Hadou with his fearsome appearance. Akuma becomes consumed with the desire to seek out strong opponents in battle and he chooses his master Goutetsu as his first test of his new-found skills because he knows his master is phenomenally strong.]
".....arm yourself now!!" The spirit kept secretly inside exploded, the cells of Gouki's entire body rose in sound with a resonating sensation. ......several hours later, Gouki's entire body in blood returned a scarlet hue, as he moved and saw the master's corpse. hadou's ultimate move "shungokusatsu," armed with "satsui no hadou" that struck the master, was complete. From then on, Gouki walked the long "path of carnage's" first walk.
"Prepare yourself!" The power hidden inside explodes out, Gouki's very cells beginning to resonate with the power. .....several hours later, Gouki, his body soaked red by blood, stirred and saw his masters corpse. The hadou's ultimate technique "Shun Goku Satsu", fueled by the "Satsui No Hadou", that struck his master, was done. At that moment, Gouki took the first steps on the long "path of carnage".
[And here is the culmination of the attack. Gouki and Goutetsu meet in their first ever death-match, and Gouki is victorious after he slays Goutetsu with the SGS. This is his first step on his dark path of mastering the fist. The real interesting part here is that it takes HOURS for Gouki to stir again and become fully aware of what has occured.]
So what we know of the SGS is that it requires Satsui No Hadou to function and actually requires so much that before Akuma, it was thought it was impossible to use it without the SnH driving the warrior mad. Of course, Akuma overcame this and mastered the SnH and, with it, the SGS. As we can see in the excerpt though, when he uses it against Goutetsu it actually takes him hours to recover from dredging up that much SnH. This explains Evil Ryu and his madness. He's possessed by SnH and its turned him into a raving, bloodthirsty lunatic. Akuma is, of course, far more self-controlled than this.
What this doesn't tell us, however, is WHAT the SGS actually is. Thankfully, Akuma actually tells us in the Japanese dialogue of SF4. When he uses the Shin Shun Goku Satsu as his Ultra he says, essentially, "A thousand strikes in an instant"...and that is, in a nut-shell, what the SGS is. In the span of a heartbeat, the SGS bares down on its victim with countless blows (or maybe around 15 or 27 if the in-game hit-counter is to be believed). Its basically a moment of pure, lethal, absolute violence, a series of attacks so quick and so perfect that its impossible to (traditionally) defend against.
And thats it. There's nothing bizarre, otherwordly or mystical about the attack...instead, its an expression of absolute mastery of the fist and pure violence. While its often been put forth that there is hell and demons and such involved, then Shun Goku Satsu doesn't literally have anything to do with all that.
If anyone is up for some discussion on this stuff, or if anyone has any counterpoints, please post 'em up! I'm going to be following this up with some musings on Mu as well, especially since understanding the SGS helps us understand Mu and vice versa!