The Surge of Murderous Intent or the destructive "Devil blood"?
POWER
The way I understand it, the Satsui no Hadou is just complete drive with lack of restraint. It does not offer any more power than you yourself already have. Anyone could do it if they had the desire.
The Devil Gene however, is a family-specific trait to the Mishimas. Except for Heihachi of course. It seems to me to be activated by the need for power. As in Kazuya climbing up the cliff and Jin needing to escape from Heihachi. However, calling it unnatural might be unfair. It's part of Kazuya and Jin afterall. But however, it's not obligatory to use it. But Kazuya mad ea conscious decision to use it and Jin's was more instinctual.
EVIL
But as for which is more evil, those with the Devil power seem compelled to destroy. Angel kept Kazuya as just a brutal importer of animals and a tyrannical head of the Zaibatsu but as of T4, he seems to be completely consumed by it. As according to his T4 Prologue, he wants to master it and has probably cast aside all good in this obsession.
Jinpachi has his natural good side corrupted and overpowered by the Devil's desire to erase all life. Devil Jin seems to want to absorb Jinpachi's power and nothing else. There's no real glimpse of what he intends to DO with the power but I guess the ending would give a hint that too wants to destroy everything. However, Jin tapped into his Devil power passively a few times, just growing wings and not really being corrupted.
The Satsui no Hadou on the other hand is just the desire to make one a better fighter and win. Sure Ryu wanted to win and nearly killed Sagat but he wouldn't go around destroying everyone else too. Akuma, the firm master of this power, does not kill and can even hold himself back. He has no longing for pointless destruction, only to get stronger.
I'd say the Devil wins hands down here.
Which would win in a fight?
It depends on the user. As I said, the Dark Hadou can be used by nearly anyone. The strong and the weak. There is no really weak Mishima so they're just adding demonic power to their own immense ability.
But of those who have canonically used it, Akuma and Ryu, it would seem to be lower than the Devil Gene. The fact also that the Devil Gene alters your body to give you new powers is a further plus over the Murderous Intent. You have to master techniques still and arent' just handed nifty laser beams.
In conclusion, i view the Devil Gene as a powerup with far more evil and more power than the Satsui no Hadou. But as I stated, it really does depend on the user.
__________________ "No, when Luke uses anger he turns into Stevie Wonder at batting practice."
Power: DG gives regeneration, wings, durability, special powers and enhanced strength, but Satsui no Hadou increase your Ki exponentially, which can be used to give the fighter immense speed and strength, and also means they won't tire as fast, and also paves the way for the strongest move in the SF universe, the SGS. I say about a tie.
Evilness: Satsui no Hadou really only means you would kill anyone to win a battle, whereas Devil can actually corrupt he user to evil.
Who would win: Pends on user, Shin Gouki has such a mastery over Satsui no Hadou, and is more powerful than any Devil IMO, so maybe him.
The Satsui no Hadou increases Ki exponentially? What are you basing this on? It does not increase any ki. The basic concept of it is just the will to kill. It is not like Ryu got a massive power up when he Dark-Shoryu/whatever'd the crap out of Sagat. He just used the move as it was meant to be...to kill. And even then it still failed to kill Sagat.
Awfully vague because Ryu also becomes weaker under the Satsui No Hadou influence.
Ki increasing exponentially + becoming weaker stamina wise does not compute at all.
All they get is the will to kill. Their chi/ki has nothing to do with it. You almost made it sound DBZ like. It is their technique that gets the upgrade of becoming killing moves.
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Actually, Satsui no hadou does make fighters stronger. All of Ryu's moves were taught to him by Gouken, who modified them to make them non-lethal, but in the process also weakened them.
So yes, Ryu was much stronger when he tapped the Satsui no Hadou to do the Metsu Shoryuken. Shotokan techniques dramatically increase in power when they are used with satsui no hadou.
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Originally, Kazuya merely made a deal with the Devil for power. This was retconned into there actually being an entity known as Devil, not THE Devil, in all the Mishimas minus Heihachi.
If it was spiritual demon thing, Kazuya in T4 would make more sense: once he died, the entity left him and tried to possess his son. This is in fact, what happened. Thus the Devil was split in half.
HOWEVER, if it's a "Gene" it's a genetic trademark and part of his very being. The idea a "Gene" can split in half and try to possess another person is just too weird to me.
And they better explain at some point WHY Jinpachi's Devil "Gene" activated when it did. WHY Heihachi lacks the "Gene".
__________________ "No, when Luke uses anger he turns into Stevie Wonder at batting practice."
It's true Heihachi > all his family members in Awesomeness.
But Jin is NOT gay. He is pretty damn cool. He makes his first appearance in a game by no-selling bullets, beating the God of Fightning and I always kicked everyone's ass with Jin.
Now...Tekken 4 Jin. You can call him gay. AFTER ALL MY TIME mastering Jin's combos and moves, he goes and uses some pussy karate. His outfit is really lame too.
__________________ "No, when Luke uses anger he turns into Stevie Wonder at batting practice."