Note: To the guest in the comments (I can't reply to guests, sorry), I think Marco is short for Marcus, but I might be wrong. Thanks, by the way, for your kind words. They mean a lot. :)
As you all know, vaults were never built to help people. But humans had nowhere else to go, what with the Great War leaving America injured badly, as well as The Wasteland being full of criminals, radioactive beasts, and backstabbing assholes that would stick a slave collar on you the moment you turned your back from them. No matter how "civilized" those in the vault were, vaults always met their awful fates. Some were doomed from the beginning, a shining example of this being Vault 77, which is still to be discovered (or has been stumbled on, but people too scared of its story to enter). This, though, is the tale of a Vault named Vault 52. It was built for the purpose of studying what would happen when one individual who had always been pushed around by other vault dwellers is given almost God-like power, and ended with said individual fleeing that Vault and causing chaos outside, leading to his eventual bloody death.
Of all the Vaults, it was the largest, consisting of 500 males and 500 females. Both genders never mixed; it was a Vault law, as if one were to be caught doing so much as waving to the other gender, they would be shot. Not to say Vault 52 hated heterosexuality; they just enforced such a strange law as The Overseer's religion stated genders mixing in any way was a mortal sin. After those conducting the experiments agreed to enforce his religious views inside the vault, it went downhill from there. It was a Vault which was still in the dark ages; white males were viewed as first class and anyone else was lower class. If a white man wished to, he could enslave any other resident of his choosing (homophobic, racist, sexist and other ignorant rules were enforced heavily too). Few did this, however, as despite the laws allowing them plenty of freedom, they had fixed ethics. Some, though, still did, and looked down upon by everyone else except themselves and The Overseer.
The individual who was driven to his breaking point was a young, black boy named Marcus. At just seven years old, he saw his family taken as slaves; some killed due to them protesting against such a fate. He kept quiet, though, out of fear of himself being shot. From there, he was trapped in a cell, forced to do nothing but sit in his cell, staring outside, his only possible actions sleeping, daydreaming, exercising, and eating/drinking the occasional scraps of food/drops of water guards slid under his cell door. Of course, these were covered in filth, uncooked, made unhygienic due to no plate under them and they tasted terrible. It was either consume them or die, though. Occasionally, a cockroach or other small insect would crawl under his cell, and distract him from the hell around him; but for every peaceful insect that came in, it would be followed by something like a Radroach which the guards would barely protect him from.
He had learned to heal himself, though, using the limited supplies in his cell. There was also a black market between the slaves; one or two would pickpocket guards (if they were caught, they would be sentenced to slow, painful death) for Stimpaks, Jet and other drugs, as well as food and drink. Some would pickpocket a weapon or decorative item like a toy car, which was a sad reminder of the fact some still lived a perfect life in this vault whilst they rotted away under the eyes of people who cared about themselves and no one else. Marcus had commonly bought Stimpaks to heal himself (being careful not to get addicted), food, and decorative items. He never attempted to pickpocket a guard though, as he had no chance of succeeding and no experience in pickpocketing. As he grew older, though, he thought of doing it.
Rebellious slaves would sometimes put grenades or other explosives they had made, found or stolen inside the guards' pockets and then set them off, despite the fact this resulted in them having their limbs removed before being thrown into a pit full of critters not strong enough to kill them, but strong enough to cause them excruciating pain and misery for the rest of their lives there before a guard took pity and landed a bullet between their eyes. Marcus, foolishly, decided to join a gang of rebels. Eventually, they were caught and Marcus was beaten (he thought he would be shot to death, but it turned out he was too young for such a punishment, which disappointed the guards). He grew a slave, was a slave, and was destined to die as a slave.
Or was he?
One night, after Marcus had bought and eaten some Radroach meat from a neighbouring slave in her cell, a box was pushed hurriedly under his cell door before he could hear the sounds of footsteps as whoever gave him the box ran away frantically, before the door to the cells eventually slamming behind them. Opening the box, Marcus found a key, 50 bottle caps, a 9mm pistol as well as lots of rounds, leather armour that fit him perfectly and a note wishing him good luck. Tired, he knew he was in no state to escape, so hid the box and what it contained under his bed and went to sleep.
The next day, he rushed to the box, grabbing the gun and the key (he had slept in the leather armour). With excited, shaking hands, he opened the door, swinging it open. He ran to the door, ignoring the other slaves who were begging him to free them, as ran out of the room right as an alarm sounded.
The vault, though, appeared to be… different. The walls were cracked, windows were smashed, and there weren't residents around. This made Marcus hesitate, and from then on looked for the exit at a slower pace. Eventually, he reached the guard house, and kicked open the door ready for a fight. What he found? The corpses of guards littering the room. Grabbing a key to a desk in the room, he found stronger armour and a Laser Rifle. Putting it on and grabbing the rifle, he walked out of the room, heading for the lounge. When he got there, tables had been turned over and there were guards waiting for him, using them as cover. After shooting the guards dead, he realised that he was no longer a helpless little slave. He was now 16, covered from head to toe in strong armour, with a weapon that could blow a man's head off with one shot. The fate of all those who wronged him previously was in his hand, and he held a strong hate and burning jealousy for everyone who lived freely in the vault up to now. He was going to push the around, not vice versa. It made him laugh with menacing excitement as he hurried to the common room.
I will not go into great detail about what he did there, but I will say he massacred all of the civilians in there during this fit of rage he is in currently, whether they had done anything wrong to him or not. He shot children, women, and men, no matter their race, size, personality, or whatever else. Blind in this fury, he murdered his friends from childhood and wasn't able to reason with them. The screams were music to his ears, and it was a disturbing sight. He had lost his mind, and was killing left and right without hesitation all around his vault. The vault had turned him into a monster. He decided, at that point, he deserved better than this vault. The world was his; so long as he could reach it. After years of abuse, he was ready to take what was "rightfully" his.
Reaching the exit, there was a guard by the control panel. Due to this new feeling of power, he happily shot the guard around ten times without letting go of the trigger. Walking over to the control panel, he opened the vault door and ran outside to freedom. In his eyes, was no longer a low class, scum-of-the-earth character with low self-esteem, no, he was now a God. Anyone who stood in his way needed to be shot directly in the brain. Delusional, twisted and delirious, he was beyond a state of mind to think that anyone was more powerful than him. Outcast rangers put up a good fight, but they were killed mercilessly and looted. Stumbling upon Megaton, he sold all of the loot from the corpses before finding the bomb. Rigging it to blow, he fled the city and detonated it from a safe distance. He wasn't even aware of Mr Burke wanting the same fate for Megaton. He ravaged and terrorised The Wasteland, the innocent and the evil both victims of him equally. After attacking group of Wasteland settlers, one of them escaped to spread the word of him. And word did spread; due to it, most avoided him if they ever thought they saw something remotely like him in appearance, and towns feared him, ready to defend themselves if he appeared. Some doubted him and brushed him off as an urban myth, and few sought out to find him. For the unlucky ones that did find him, it was inevitable; they were not going to return to town.
Note how I said there were still some stronger than him? Well, a group of ex-Enclave scavengers had been watching him from the moment he stepped out from the vault. They were passing Vault 52 when they heard the door open, so they fled to a nearby rock to see what was emerging from it. And then, out stepped Marco, ready to face the world. They were low on supplies and pretty battered from when a gang of Raiders had just attacked, so they ran. From that point, they followed him, and for every bit of armour that he left on bodies, they took for themselves. Yes, it was weaker than Marco's, but it was also a gang of three, well trained, devoted individuals against one vault dweller who couldn't think straight, plus the countless bounties gangs and towns had set over Marco's head. Anywhere from 1,000 caps to 10,000, the trio wanted him dead and wanted the money. And when they were ready, they appeared.
Marco was alerted when one of them coughed for his attention. He turned, and fired at all three using his V.A.T system. It was incredibly lucky none of them were hit, and Marco was confused. Usually, they would be dead by now. But it had caught him off-guard that he had missed his targets. Angry, he charged, unable to think that he could be killed any moment. As the ex-Enclave group pulled out their weapons, they fired endlessly with all the ammo they had. They couldn't see what was happening, as the bullets kicked up a large amount of dust, not to mention the minigun aided this hugely. After the dust had disappeared, though, they saw his body, riddled with bullet holes and blood running, sometimes spurting, out of them onto the floor.
The test had failed, as they were unable to gather information on what Marco had done outside of Vault 52. They knew, though, that he went insane in the vault due to the sudden power he had gotten and was blind to anything but his objective of getting revenge. It had caused the deaths of hundreds, maybe thousands, that didn't deserve it, all because of a pointless test carried out by sick individuals. The man who had given Marco the crate? Nobody knows who it was; the vault housed limited CCTV, and the camera in the room full of slaves had been smashed by a rock thrown at it, nobody bothering to repair it. Marco was dead and now the slaves that helped him were now doomed to slowly starve to death because of him once they ran out of supplies, probably going insane too and turning on each other.
But then again, vaults were never meant to help anyone.