If you excuse me I am gonna do some speculating here, as after watching the interview with Nicolas Obregon I had some thoughts.
I am going to preface this by saying that I do think that both the military and skater theory have some merits to it but I wanna look at it from another angle.
1. The knife. I think it's interesting that the perpetrator decided on a sashimi knife. For one I think its something that a non asian person would be unlikely to own and also because of how inadequate it is for murder. Most self defense / combat knives have very obvious grips often with a hilt specifically to prevent slipping from a stabbing motion. The sashimi knife doesn't have any of that, it has a rounded slippery handle and is made from hard steel which can easily clip. I feel like someone in the military would know why combat knives look the way they do.
2. Hanging around. I think the reason why the perpetrator decided to hang around was because of the blood loss. After the adrenaline started wearing off he probably started feeling dizzy, and just the same as after a blood drawing he got something sweet to get his blood sugar up back to normal levels.
3. Location. One reason that makes me question the randomness of the attack is the chosen location. If I wanted to do a quick purge style killing, knowing I will be flying back soon after then I think choosing a 2 floor house with another right next to it is a pretty terrible choice. You wouldn't know how many people would be inside but you can pretty safely assume its more than 1 so you start with a disadvantage. You also have no knowledge of the floor plan or if the people inside have anything near them that they could use as a weapon. Moreover having another house that is occupied right next to means that you are much more likely to get caught. Of course I am not saying it's something impossible for someone to choose, but I just don't see why if you were some random military brat and wanted to kill someone right before going home would you choose a location that puts you at severe disadvantage as opposed to just bumping into someone on a dark street or near the park. Moreover a 1 night thrill kill and then living your life after as if nothing had happened is not a common thing. I think what is more likely is that the perpetrator was someone who had interacted with them before, maybe even multiple times just like with the Namiko Takaba case. Maybe even years before and carried some weird grudge specifically against them.
4. The clothes. Although the sand and korea only sneakers are interesting and as I started I do think the military theory is a possibility but Tokyo is also full of second hand stores. I don't see why the perpetrator couldn't have just bought something at one of those places which wasn't properly washed.
5. How did he get home? If we assume he wasn't military and also didn't just randomly choose the house then its very likely he lived nearby, maybe less than a hour walk away. If he was a local then he would have knew the location of the nearby koban too, knowing to avoid it.
6. The time. I think its worth mentioning that japanese people don't have that many holidays but Dec 30 would be a time where most people would be on vacation as it is the new years holiday.
7. The killers personality. What if the reason why he was so careless, going through documents, only taking some of the money, going back to the bathroom window to check outside, leaving the toilet unflushed wasn't because he was so sure of himself but because he was actually panicking in a half delirious state?
8. Using sanitary pads to stop the bleeding. I honestly do not think this points toward military. You cut yourself, you are out of bandages what do you do? You go to the bathroom and try to look for something sterile to wrap around the wound.
So to sum it all up, I really do think that one possibility is that it was someone that one of the family members knew at least a little bit and that person had a grudge specifically against them.
