As the title indicates I'm looking for a manga about a man who was about to commit suicide in an abandoned building. He didn't have much to live for and had no outstanding qualities. Unfortunately, he stumbled across a serial killer in the middle of cleaning up his crime. He picked up a gun and shot at the serial killer who also fired back. This incident caused them to switch bodies, although it's presented as speculation on the protagonist's side at this point.

As for the serial killer, it turns out he's a very handsome man who is successful at his job as a police officer. The protagonist doesn't know he's in the body of a serial killer until he goes to the man's house and searches the basement revealing dismembered body parts and torture/body disposal instruments. He also finds a child, but manages to convince her that he's not the serial killer and explains the situation to her. They sort of team up to survive against the serial killer who has taken over the protagonist's body.

Although the serial killer's original body is very high specced, able to stop a doped up criminal in a single move, even after the serial killer's mind takes over the the protagonist's less athletic body, he's still able to put it to frightening use with his skills. In a significantly weaker body, the serial killer takes down a larger man who is a head taller, mostly muscle, and works as a thug for a loan shark.

Yes this is it. Thank you.