The article that talks about what happened says that the main character was a 20 year old college student at the time. So he's 27 at the time of this story. Which is really bad for the main character. He's basically lost all of his young adult years, never finished college, and he was an adult at the time he committed murder. His record is known, and, in Japan, once your a criminal, your always a criminal.
What a lot of people commenting on the story don't understand is that in Japan, being a criminal basically marks you for life in Japanese society. You are always a criminal from that time forward. You won't be able to get a job, you won't be able to get a home (nobody will rent or sell to you), nobody will want to marry you. The main character is basically persona non grata as far as Japanese society is concerned. He's screwed basically. Which is probably weighing on him as well. The main character will basically never be able to take care of himself, so he'll be reliant on his mother for the rest of his life. This is why Japan has one of the highest criminal recidivism rates in the world. For many ex-cons, going back to crime is literally their only option to support themselves. Many simply do go on to commit suicide.