But what purpose did the Virus serve exactly, aside of a very effective weapon against his Reploid brothers, if he started his rebellion on his own free will? Did it trully affect his personality or did it nourish the already growing darkness of his heart?
I would say that becoming infected with the Virus made Sigma realize what he could do, he was given the ability to whatever his heart and mind desired. But all of this is first and foremost driven by his tattered pride, leading into his desire for power. In the middle of this entire process, he meets X and his strong justice makes him realize the imperfections of the world. When all these factors come together as Sigma merges with the Virus, it means the Virus made him see unlimited potential to grow stronger as he had the power to cast the world into endless war to invoke the evolution of all Repliroids.
X8 is right on the mark with "Sigma rebelled of his own free will", solely because he is the Virus and the Virus can't corrupt itself. A personality change is doubtful, as that is disregarding both X and Zero's non Virus influence on him. Furthermore, it was remarked that Sigma does not worry much, whereas X worries too much. VAVA was said to be similar to Sigma in that he can become irregular of his own accord. Which would lead to the conclusion that the likes of VAVA and Sigma have no compassion because they do not worry.
You know, what I'd find really helpful is a comprehensive diagram about all the major Irregular indicents (read: games), causes of the Irregular revolts (Virus, plain conspiracy...), the Virus' variants and mutations, etc. It'd be like those global warming charts, just useful and I could actually hang it from my wall.
Maybe not a diagram, but here's a little timeline:
-Wily is constructing Zero and the Sigma Virus.
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-Wily puts the Sigma Virus inside of Zero to make him an irregular.
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-Zero fights Sigma, Sigma is infected with the Virus. > After several months, the evil Virus' hold over Sigma
became complete and Sigma started his rebellion.
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-Wily who died in the interim is brought back by the Virus anytime before X2 in some unexplained matter.
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-Magne Centipide is spreading the Irregular Virus from the Central Computer across the world.
-Central Computer has several computer-aided materializations of Sigma Virus. (Sword and Sigma face)
-Sigma Virus is in one scenario used to awaken Zero.
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-Techno is infected with Sigma Virus and realizes Sigma has the power to manipulate Repliroid hearts.
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-Doppler develops a working anti-Virus but is infected by Sigma. He creates a fake utopia based on a fake anti-Virus. After his defeat he reveals Sigma is in truth the Virus itself. Sigma tries to infect X to take over his body, but is stopped by either Doppler or X using the real anti-virus. Sigma's program is fading.
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-Tackione, who is a Limited clone of Doppler, produces Sigma Chip which holds the key to Sigma's immortality.
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-Enemice HL, a creation of Doppler uses the Sigma Chip on himself and becomes Sigma. Sigma is defeated by X and ReturnX's merger. ReturnX seemingly claims the power of immortality.
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-Sigma reappears as the mastermind behind Berkana and Gareth.
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-On Sigma's orders, Dynamo spreads the Colony Virus on Eurasia and makes the colony drop.
-Sigma purposely sacrifices a body of his to spread the Virus all over earth. Massive outbreak of irregulars all over the world.
-Zero witnesses power boosts from the Sigma Virus purifying his body and trying to turn him into an irregular.
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-Sigma's resurrection has slowed down because he spread too large an amount of the Virus.
-From Eurasia's remains, the Colony Virus and Sigma Virus merge together to form the new Zero Virus, which purifies Zero's body and possibly awakens him.
-The Zero Virus causes the 0-space, a merger of cyberspace and the real world. Programs from cyberspace are materialized in the real world.
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-The Zero Virus disappears with the destruction of the 0-space.
-Sigma is completely destroyed by Zero. Zero has finally defeated the Sigma Virus but dies shortly after.
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-Gate finds a piece of Zero's DNA and is infected by the Sigma Virus spreading from Zero.
-Gate resurrected Sigma from complete destruction by comparing Sigma Virus DNA with Zero DNA and triggering Sigma's revival program.
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-Cedar obtains Sigma Virus Mk2 and attempts to use it as a bargaining tool. However, that kind of protection is useless toward Axl and Cedar is killed by Axl. Axl's success attracts Sigma toward Red Alert.
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-Sigma manipulates Red and by the time Red realizes what's going on, he has taken complete control over his comrades using the Sigma Virus. He uses them to force Red into doing his bidding.
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-New Generation Repliroids appear with complete and total immunity to all Viruses.
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-Sigma is once again completely destroyed, this time on the moon using his strongest body.
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-The Sigma Virus continues to exist. Marino becomes a technology thief in order to find a cure for Sigma Virus.
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-Zero learns he's the origin of Sigma Virus and that he's spreading the Virus during missions. He admits himself in a research institute to get rid of something inside of him.
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-From Zero's Sigma Virus data the Sigma Antibody Program: Mother Elf is created. Project Elpizo, Birth of Hope. The Mother Elf has the ability to revert irregular programming to its original form.
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-X's uses the Mother Elf and the war quickly ended.
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-Dr. Weil awakens the Sigma Virus dormant inside of the Mother Elf to curse her. She becomes the Dark Elf. The Elf Wars begin.