Barry's story began when he was eleven, when his mother was murdered. When nobody believed his dad was innocent, that he didn't kill his mother, he lost all faith in humanity.
The foster system ruined him completely. He was stuck in some seriously messed up foster homes, before finally getting adopted by the West's. Iris knew Barry was different than he was before, but she tried to ignore it. She ignored it as much as Barry tried to be normal.
The first time Barry actually killed someone, it felt wonderful. He was 18, and just got into college. Barry knew how to cover up all signs it was him, but he didn't know how to frame someone else yet. He would learn, though. He wanted someone to feel his father's pain.
Common belief is that Barry became a CSI for the sole purpose of proving his father innocent; that was only half the truth. See, Barry needed a way to cover up his crimes, to make the people who uphold the law believe he was too… innocent… to kill anyone. Besides, going to a CSI college, majoring in crime scenes? Perfect way to get ideas, and learn how to cover up his tracks.
Joe and Iris never knew. Barry never revealed anything to them, nor would he stay the night in their house. He felt bad enough, them thinking he was a good young man. Even if they didn't know, they didn't deserve to have a killer sleeping under their roof.
As the years passed, more bodies dropped. Barry made himself known, or some sort of alter ego of his known. He didn't have a name, but he was definitely feared in Central City. Every time he dropped a body, and severed, burned, or hung it, he would end up in the newspaper. Every time he saw a picture of his art in the paper, it made him feel proud.
Joe made it known to Iris and Barry that he hated this guy. He gave both of them lessons on what to do if he approached either of them, and he even gave them guns. Little did Joe know, he gave the serial killer a gun.
Now, Barry wasn't completely fearless. The one thing that constantly had him on edge was the Starling City vigilante coming to... visit him. He may have had others killed, but he didn't want to be one of the bodies in the paper.
But when he found out who the Starling City Vigilante was, Barry knew he had the upper hand. Not only had he gotten on the Vigilante - no, the Arrow's good side, but the Arrow also thought of him as a weak nerd from Central City. It was perfect.
What wasn't perfect? Getting struck by lightning. He couldn't worry about this while he was comatose, but what if people put the pieces together? Local CSI gets struck by lightning, and around the same time bodies stop dropping.
When Barry woke up, Central City was more relaxed. He noticed that people would go out at night more, and no one was paranoid. It was like he lost his grasp on the city. This wouldn't stand.
Of course, Barry wasn't stupid. Of course, he was hungry and full of bloodlust, but he couldn't go right back to dropping bodies as soon as he could. That would basically scream, "My name is Barry Allen, and I am a cold-blooded killer!"
Then he discovered he had super speed. Naturally, he went to Oliver, he wanted the Arrow to believe he was a hero now. Had to keep him under his finger. Then, he found out Cisco made a suit, and it was perfect! Not only did one of his alter ego's have a suit now, but he could make the other one a friction proof suit and get more… creative with his ways!
Those that knew Barry was the flash, he didn't have to worry about with his… other job. Why would a superhero be a wanted serial killer anyway?
God, he really needed to come up with a name for himself.
That's where he is now. Having just stopped a tornado, the relief of saving a life wasn't /nearly as good as taking one. He had to hide his anger. If Barry didn't get a life on his hands soon, he would surely go insane.
He was only awake for a day, people would know.
Not if they don't find the body.
Barry mentally thought out what he would do, while Singh was talking to him and Joe. He made it look like he was checked in, though. He would not allow Singh to give him a medical leave. He needed work. It was one of the reasons he was still sane, if you could still say he was.
Once Singh was done rambling, and told Barry to go home, Barry gladly complied. He could feel his heart rate increasing, his sanity slowly going down. Killing was like his drug, if he didn't get his fix in he would go through withdrawl.
Barry sped to his apartment, he had to get his mask, before leaving again. He didn't take anything with him, though; this was simply a grab-and-go mission.
Barry got into an alleyway in a not so crowded part of town, and waited.
And waited.
Finally, a man who appeared to be in his late thirties walked by, and Barry pounced on him. He covered the man's mouth with his hand, and dragged him into the alleyway, before flashing him back to Barry's house. Normally, Barry wouldn't bring any of his victims back to his house, but he had the time to clean up their messes now.
He sat the man on one of his kitchen chairs, and zipped around his house to find a rope. He knew he had one; everything would be useful eventually. He tied the man to the chair, and duct-taped his mouth.
"It's been a while since I've been around, huh?" Barry asked, twirling a knife in his hand. "Yeah. I was… out for a while. Turns out people aren't as afraid of you when you take a nine-month hiatus. Fucking lightning," he continued, while the man tried to squirm out of the chair. Barry chuckled. Even in the man escaped, there was no way he would get away alive.
"Don't bother trying to escape. See, over the nine months, I acquired a new… superpower, so to say. A speedy one at that," Barry explained. "Even if you get out of that chair, there is no getting away." Barry walked up to the man and dragged the tip of the blade to the struggling man's arm. He then grabbed the duct-tape, and taped the man's eyes.
Dragging the blade further down, Barry watched in admiration as blood droplets popped out. It really has been too long.
"Alright, I'm tired of waiting," Barry said, and with that, he plunged the knife into the man's heart, once, twice, three times, before he started laughing.
His heart rate was too high.
He kept laughing.
The body fell limp in his kitchen chair.
He laughed even more.
Eventually, he stopped, and he cleaned. If you were to go into his apartment, it was as if he never killed a man into his kitchen.
He sped the man to the lake bordering Central City, and dumped him there. Another perk of superspeed: it provided an easy body disposal.
Barry sighed in relief as he looked at his clean kitchen.
Man, it really has been too long. Starting tonight, his mission to take back the city was underway.