Hello i created an account just because i'm so desperate to find out WHAT exactly this show i watched was. My mom is japanese so she always watched japanese dramas in the backgrounds and i would sometimes watch along. There's this one episode that i can vaguely remember but cannot find for the life of me and i thought you guys might. I'll list out the plot points that i remember: 

It was one episode out of a series, not the entire plot of the series. I believe it's a crime drama very similar to tantei gakuen q, where it's a small team of detectives (maybe kids?) that solve mysteries, and this episode was one of them. 

There's a guy who has a limp and doesn't ride the elevators because he's claustrophobic, he had an abusive dad when he was younger that would lock him and his little brother in a closet. When they are grown up the brother ends up killing this dad, and the guy protects him from persecution or something. In present day, the brother goes around killing people (?) and the guy goes back to the crime scene to clean up the evidence so the brother doesn't get caught. At the end of the episode, the brother has a knife pointed at the protagonists and the guy is like hey put the knife down, but the team is like "you're the one holding the knife". It's revealed that this guy actually has multiple (or split) personality disorder, and he's been the "brother" going around killing people present day all along. When the brother killed their dad, he also killed himself and the trauma of that made the guy create a reality where his brother is still alive in his head. The team realizes this when in security camera footage, i think a school? The guy isn't walking with a limp, and they believe it's because he's his "brother" at the time, thus not having the limp. 

This has been killing me for YEARS now!!! I finally asked my mom what the heck this show could've been and she said she has no idea. Since it's japanese, i can't find transcripts of it ANYWHERE on the internet :( :( :(. If anyone out there has any idea where this could've been from, i would love to hear!!! 

Wow idk what that is but it sounds really interesting! If anyone finds out, please PM me! 
This might be from 1 of the Kindaichi Shounen no Jikenbo series.
There are about 4 dramas that have been adapted from the series.
This episode may be from 1 of these dramas.
I think it's from Boss season 1 episode 09
http://mydramalist.com/175-boss
what you described is identical to that episode but the team of investigators are adults not young kids.