Omniverse: The Forsaken Child

*Note: This is a slow phase story.*

Kei is a child abandoned by his parents or he was told so, sold to the military after discovering his innate talent for adapting into anything and high intellect.
Kei was trained at a young age to be the perfect killing machine, unfortunately, for them, they couldn't make Kei abandon his emotions, unlike how his parents abandoned him.
One day, Kei and the others had enough, though a child, Kei is very mature and smart.
Kei together with the other children tried to escape but was caught, the officers used one of the children as an example, Kei best friend who taught him about how the outside world was. His best friend was killed.
Kei was enraged but kept the boiling rage inside him.
Years later, Kei had turned 15 and the people responsible for the facility thought that Kei had become the very thing they desired, a hound dog, but never did they expect that the hound dog would bite its owner.
Kei who knows everything about the facility, enacts his plan.
He killed everyone in the place, even his friends who had become literal hound dogs and tried to defend the 'owners' we're not spared.
Kei after killing everyone, erases all the files and information about him and everyone else, turning himself into a ghost (figuratively).
Kei joined an orphanage aiming to learn the workings of the real world bit by bit and there he began to live the life of a kid until one day the government got wind of him even after he had erased all information regarding his existence.
So Kei escaped but all of a sudden, he was caught in a dimensional gap and found himself in the world of Danmachi with the power of Tensura.

Worlds:
1. Is it wrong for a girl to pick up girls and have Yuri harem in the dungeon?

Upcoming Worlds
1. That time I got reincarnated as a bishoujo slime