WHY did he kill him? (spoilers)

I felt Mamoru killed his boss and the wife in order to save Yuji from doing the deed. Yuji was most likely going to do so (since I felt he hated the boss more) but Mamoru wanted to protect him (Mamoru in Japanese means "protect") and sacrificed himself instead since Yuji still has a future ahead of him. Since this film is about the young and old (or the generation gap), the whole idea of saving the young (Yuji) for their future is the older person's (Mamoru) responsibility.

I don't know why Yuji was in a rage to get his CD but I felt his rage was just pent-up anger from the fact that Mamoru quit his job, he couldn't find Mamoru, the bowling alley was closed, and things were just going bad for him at the moment. Then he just suddenly thought of his CD...one of those moments. At least that's how I interpreted it.

I don't think you'll get a satisfactory answer. At best it is symbolic, wanting to retain their youth, and not have it wind up as hackneyed as this old man coming over and wanting to tell them what a wild guy he was in his twenties. That old man, indecisive in his furniture placement, excited by a ping-pong match, and wanting to siphon some youth off of his workers (via that CD), that old man was the "bright future" that threatened Nimura/Akita.

Got to believe there is a "move forward" that doesn't involve kill those who "wait", personally.

I felt like Mamoru killed the boss because Nimura was about to become a permanent employee and start living a cookie cutter boring life. In a way Mamoru's murder of the boss and his suicide are symbolic of Mamoru's advancement of Nimura towards his future and identity. Otherwise Nimura would've ended up just like everyone else, just like the boss.

In giving the jellyfish to Nimura, Mamoru was giving Nimura his destiny. The acclimatization of the jelly fish to freshwater is symbolic of Nimura adjusting to the world and coming to grips with it and it is finalized by the release of the jellyfish into the city system when Nimura breaks the aquarium.

This is a deep and subtle movie that will not please most viewers because it requires a lot of thought and analysis on the viewer. But for I found it exceptional both in its cultural representation of generation gaps as well as the effective use of cinematography to portray the dreary disillusionment that people go through.

"The sleep of reason breeds monsters"

Mamuro knew that either him or Nimura would end killing his boss and wife because they both got angry at them for several things that for most of us doesn't really justifies to kill anybody.
These things are: how their boss' wife used them to move furniture from one place to another again and again. How his boss intrudes in their apartment, sits and turns on the TV and starts clapping and cheering like a fool while watching a ping-pong match, seems that was too much for this obscure boys. Also their boss insisted too much on Nimura borrowing him a CD. Finally he --the boss-- offers to both of them a long-term contract in what seems the boys considers a job without future... certainly that's not the "Bright Future" that Nimura is constantly dreaming about.

Now if you think that those are reasons enough to kill a family, that's another story. Apparently japanese teenagers and young adults use to blown away quite rapidly and for no obvious reason. Pretty much like those kids in Columbine or the dude who killed those students in Virginia.
Nobody really knows why the hell they does this kind of awful things, but they keep doing it.

I think both of the characters see nothing positive about life. They are literally bored too death and see absolutely no hope for the future. The miserable lives they are living will always be miserable. I think they strongly resent their boss, because he is finding some kind of enjoyment in life, even though his life is miserable as well. I think they have become so despaired that they think very little of killing him, because life is so boring and pointless, that, murder, is something to do and if they are caught, who cares, their life won't change that much, and if they die, so what?