When Deku stepped out of the portal, he was met by a sight he never expected yet made perfect sense. The hero Eraserhead had been thoroughly beaten. His face was a bloody mess and multiple of his bones were broken based on the fact that arms don't bend outwards. He was being held down by the Leagues 'secret weapon', a hulking beast of a creature called Nomu. It was as big as All Might, if not bigger. It had deep black skin with multiple scars and tissue tears. The only clothing it wore were some tan shorts that ended at his knees with some skull knee pads. AFO had explained that a Nomu was a creature that had multiple quirks infused into them. This Nomu possessed shock absorption and increased strength, speed and regenerative capabilities. It was perfect for taking down a hero that focused solely on strength. A hero like All Might, for example.
Kirogiri and Shigaraki were standing behind the Nomu and Shigaraki was excessively scratching himself, indicating that something had gone wrong. "What happened?" Deku asked as he approached them, waiting for Shigaraki to explode at being told what to do by Deku. To his polite surprise, Kirogiri responded with, "despite my efforts, the student with the speed quirk managed to escape. He's no doubt bringing reinforcements as we speak." Shigaraki pointed toward Kirogiri and said, "If you weren't a warp gate, I would kill you right were you stand". Knowing that a cornered animal is the most dangerous animal, Deku figured he would do some recon to put some distance between him and Shigiraki... and hopefully be away from the main villains when backup came. He did not want to be their priority.
As he was patrolling, Deku noticed something. Or more accurately, two someones. A small boy with purple... orbs... on their head, and a girl in a green outfit and a long tongue. Both were hiding in the Shipwreck Zone and were observing as events played out. Deku sauntered to their location, pretending that he didn't see them. As he approached, he could see them out of the corner of his eyes. He could see their muscles tensing as they prepared to flee. However, as he arrived at their location, he turned his back to them to shield them from the sight of other villains. "Hey, you two, get out of here," he whispered out of the corner of his mouth. Understandably, they were surprised that a villain was sparing them. "Why are you helping us? You're a villain," the girl said. "I am a villain, but I'm not a monster. I don't kill for no reason. When I do, it's either because I have to, or to send a message. Killing you doesn't accomplish either." Feeling like he had given a sufficient explanation, he continued to walk around the separate zones to 'do some recon'. He heard the students swim away behind him, silently glad they wouldn't get involved. After a while, he decided to head back to ensure that Shigaraki wasn't going to overboard.
As the villains entered his vision, there was a large explosion at the entrance to the USJ. Deku turned, already knowing who had caused it. Lo and behold, it was All Might. His ever-present smile now replaced with a terrifying scowl. "It's all right students... because I am here!", he proclaimed, the scowl not moving an inch. Shigaraki, who was probably about to do something rash and stupid, muttered something about 'getting a continue', before announcing, "I've been waiting hero. You trash of society." Other lower-class villains began to ready themselves, preparing to take on the Number One Hero. Deku knew all the quirks at the Leagues disposal and instantly knew their odds of success. Approximately...0%.
Before the words were even out of their mouths, All Might was on them like locusts on a wheat field. He effortlessly defeated any lower class villains and retrieved Eraser Head from the Nomu's clutches faster than the eye could see. He even managed to retrieve Bakugo and Kirishima from the Collapsed Building Zone. Deku couldn't help but smile to himself. This was his childhood hero after all, still being a role model despite being his opponent. He returned the captives to the group at the entrance, before turning his attention to the villain ringleader in front of him. "Carolina..." he began, dashing towards Shigaraki, arms crossed across his chest, "...SMAASH!" he yelled, lashing out at Shigaraki. The shockwaves it produced were so strong, they caused the nearby Shipwreck Zone to produce multiple mini-geysers. However, it had no effect on the Nomu, which had dashed in front of Shigaraki at the last second and absorbed the blow. As expected, it had no effect. This was what Nomu was made for. But that evidently wouldn't stop All Might from trying. He and Nomu began rapidly trading blows, faster and faster, each punch more powerful than the last. The shockwaves alone were enough to blow away any villains that were too close to the two. And yet, despite being weakened and despite Nomu being the perfect being to defeat him...
All Might still won.
With the finishing words of "GO BEYOND! PLUS... ULTRAAAA!", he punted Nomu so hard he broke through the walls of the USJ, two nearby clouds and beyond until he vanished from sight. "In my day... I would have defeated him in five punches," All Might proclaimed, turning his attention to the villains who now had no defense against him whatsoever. Even though it was obvious he was running out of steam, All Might still intended to defeat Shigaraki and the League of Villains there and now. All Might leapt towards Shigaraki and Kirogiri, fist pulled back and ready to blow. As he approached he swung his arm forward, intending to knock them unconscious and into submission for the next couple of hours. Instead, his fist met only air as a third villain pushed them out of the way at the last second.
"He pushed them out of the way," All Might thought, "despite the speed I was going at. How did he do that? His quirk must be powerful". All Might tried to take a step forward, but his body wouldn't respond. Realizing he was at the end of his chain, he stated, "What's wrong villains? Not so cocky without your Nomu, are you?". He spoke with such confidence and dignity even Deku almost believed it. He saw the signs; the quivering of muscles, the slight wavering in his voice, the reluctance to do... anything. He was at his limit.
Deku had a choice. He could either tell Kirogiri and Shigaraki and kill All Might once and for all, or they could retreat, leaving All Might alive to fight another day. Despite being a villain, Deku wasn't stupid. He knew that reinforcements would arrive any minute and that if they tried to kill All Might, they would most likely to be gunned down by them. He also knew that he was just telling himself this as a form of self-sabotage. None of this bothered him, he was just trying to figure out the course of action with the highest likelihood of survival.
Whilst he was thinking this, Shigaraki and Kirogiri already decided on a course of action and rushed All Might, very nearly decaying him. Unfortunately (as Deku predicted), reinforcements chose that exact time to show up. The hero Snipe shot Tomura five times: once in each hand, once in each leg and once in the chest, before Kirogiri blocked the remaining bullets with his vaporous self. Deku leaped towards Shigaraki, holding him upright as Kirogiri teleported them away. "All things considered, that went pretty well," Deku said, before Shigaraki yelled at him to "SHUT UP, DEKU!" And with that, they were gone.
So his name's Deku, hmm?, All Might thought to himself as the villains disappeared. He was strong, I'll give him that. He had to be if he could out-speed me even slightly. With such potential, what could possibly have turned him to a life of crime, he thought, completely unaware that the reason was unconscious not 50 feet away from him.