Two things from this case that give me the most consternation is the killer's motive, and the actions (lack thereof) from the family next door. I've always felt there's a connection with these two issues. While it may be crass, I mentally refer to is as my "Mike Was A **** (handful) Theory"
Nic's reported that Mikio's coworkers are unwilling to talk about him. TMPD isn't releasing any personal information about Mikio. And, the Irie's are pretty tight lipped. Is this all in an effort to keep from Mikio being victim shamed or blamed?
In regards to the Irie family's silence. Based on Nic's soundproof expert, and common sense, it's safe to assume the family heard more than they reported. I wonder if Mikio occasionally lost his temper, and loud sounds next door weren't 'unheard' of? Maybe they thought it was another family quarrel? Were there previous instances where maybe they intervened with family arguments, and it only further exacerbated the situation?
I'm uncertain of the time frame when the soundproof materials were installed between the homes. However, we know Mikio paid for it. If it was installed after the homes in the area were being sold, and Mikio was going to be selling his/it was going to be torn down....why? Plus, it seems clear it was ineffective. I'm wondering if Mikio installed it as a show of face, out of frustration, to send a message concerning privacy, etc.?
Mikio possibly had "discussions" with whoever was parking their car too close to the home. Was the car owner possibly the POI? Was he parking there to access the park?
The skater angle is one that keeps coming up ad nauseam. However, I wonder if the killer witnessed Mikio arguing with them over the noise? Did Mikio become overtly angry? I've always felt there's 'something" to the skater angle. Obviously, so has TMPD, based on their intense investigations of skaters. But I don't think the killer was necessarily a skater. Based on the proximity of the tennis courts to the skate area in the park, maybe the killer merely witnessed the interaction?
Regardless of how the killer may have been involved with the skater argument, if you combine this with the car issues, you can start to see how some rage and anger might build. Especially in a pubescent teen, and doubly especially one with a military upbringing. To me, it presents a logical motive.