"I am Shadow...the Hedgehog. I was created to bring order and justice to the humans. With the power of these emeralds, I'm going to destroy this damn planet. This is who I am!"
The Black Arms attack on G.U.N. Fortress resulted in a horrendous bloodbath, the aliens didn't discriminate in their slaughter. Troops fell like toy soldiers, armored mechs were tossed aside with ease, not even civilians were spared.
It wasn't a battle, it was a massacre.
Shadow himself acted as the vanguard of the assault. With the Ultimate Life Form on their side, victory was all but assured for the Black Arms. Before communications went silent, reports came in of a black hedgehog carving a path through the fortified base. Some even mentioned him laughing as he killed his enemies.
Indeed, Shadow took great pleasure in his work. Black Doom's words and his recently recovered memory of the tragedy on Space Colony ARK fueled his newfound hatred, empowered him, and warped his Chaos powers into destructive mayhem. Being able to let loose on these hypocritical humans, words could not describe the elation Shadow's slaughter provided. Finally, revenge for Maria and Professor Gerald would be had.
At the climax of the attack, the leader of G.U.N. and a disappointed Sonic the Hedgehog acted as the final line of defense for the last Chaos Emerald. His rage reaching its fever pitch, Shadow's wrath in the form of negatively charged Chaos powers annihilated them both. With all seven Chaos Emeralds in Shadow's possession, and the the deaths of Sonic, G.U.N. Commander, and later the President of the United Federation, the planet became easy pickings for the Black Arms. Tails, broken by grief, rallied Sonic's friends and launched a last ditch effort to defeat Shadow. All were slain, even his former teammates Rouge and Omega. Shadow perceived Team Dark as traitors, their deaths fanned the flames of his hatred and further empowered him.
Hope became a distant memory on Earth, a fairy tale parents told their children as they awaited the end.
Black Doom's army swept across the planet, destroying nations and enslaving the population as it went. Well, enslaving anyone Shadow missed, at least. With him leading the charge, few survivors were left. The few that managed to escape brought with them grim tales, detailing a monster leaving blood and fire everywhere he went. In Black Arms held territories, city sized internment camps were established as a means of transitioning humanity into their new role as cattle. If you weren't performing slave labor, you became food for the next generation of Black Arms.
Shadow's hatred for humanity never eroded. If he wasn't fighting on the front lines, he toured internment camps and hosted mass executions. When the conquest officially came to an end, the latter became his only activity. Black Doom allowed this, both out of pride for his new General, and out of fear of reprisal if Shadow didn't get his way. He would never admit this, though. There would be times when Shadow questioned his new role, begin hesitating to bring the hammer down on pathetic humans begging for their life. Maria would come to the front of his mind, and the black hedgehog reminded himself of the horror humans inflict on each other for petty reasons. Shadow needed to keep feeding his hatred, if it abated for even a second then the guilt would destroy him. Then these humans would go unpunished. The Black Arms occupation was business as usual, they weren't interested in delivering judgment. Only Shadow could deal the necessary punishment.
They deserve no less.