I used to run another blog on wordpress back in the day (fuck wordpress), and I wrote an entry about a murder case where Belgian police found a human torso and 2 thighs laying ina park, next to it was a note that said "watashi wa KIRA dess".
Facepalms were had, not just for the blatant mispelling but for the uber lame Death Note copy cat. Turns out recently the case was solved. This is good, because still today my blog gets some weird ass comments from Kira wanna-be's. Go read that link, you'll lol.
As I was saying, apparently the four people involved got into a fight with the man that was murdered, my guess is he was a shitty Dungeon Master, those piss me off too. Well, regardless, they killed him. From the way the article was written, leaving the mispelled note was just a side thought on their part.
As in, they killed him, cut him up, and then was like "Hey, man. We should totally leave a note like Raito did, that'd be bitchin."
Sounds like it was more premeditated than that. Murders don't usually get spur of the moment awesome ideas like that, I know, I've watched enough Law and Order SVU to know.