Shamo ~ Izou Hashimoto & Akio Tanaka
Shamo is a fighting seinen manga about a boy named Ryo Narushima who was a 16 year old bookworm in High School and getting ready to go to Tokyo University. One day he snaps and brutally murders his mom and dad with a switch blade, leaving his sister to watch as he cut them up. He's sent to a juvenile reformatory where he meets Kenji Kurokawa, a prisoner from a different jail who comes to the reformatory once a week to teach karate to the boys. During Kenji's lessons Ryo finds that he has a real talent for karate. Thanks to a law that protects minors he's released from the reformatory two years later at the age of 18. From there Ryo's life changes forever as he sees that he's shunned from society and left to fend and survive on his own.
This is probably my favorite seinen manga to date. The storyline is very psychological leaving you to wonder what the hell is going on in Ryo's mind. The fighting can get very bloody and gruesome, but the artwork is simply stunning. The manga is currently ongoing with 25 volumes, but sadly only up to about half of 18 has been scanlated so far. The latest to be scanlated was chapter 186 by Manga-Sketchbook.
Shamo also has a live-action movie currently in the works. Go here to find some info.