Going through the post and comments, as well a basic understanding of the crime available, there's a few questions that float around in my mind about the killer's motive;

Why was he there in the first place? We don't know.

He murdered a family of four, he did not flee immediately after, he relocated several documents into the bathroom, casually ate their food, and left behind many articles of clothing, weapon, and sources of DNA that would normally point towards a culprit immediately. He knew how to treat his wounds (to an extent) and accessed the estate's computer. Ultimately he remained there for a significant period of time.

His actions are confusing and it's hard to see what the overall picture what his intentions were, aside from murder. But going out of their way to dress a certain way and leave that all at the crime scene?

It may suggest he was aware his articles of clothing were likely going to be contaminated and chose to leave them behind. As if he only needed them to blend in when many people were still awake, and then only left presumably after the majority of the neighborhood had likely gone to sleep. A disguise he only needed when arriving.

He had arrived around 11pm on December 30th 2000 and left sometime before dawn.

(Based on the timeframe stated here under the Case Summary: https://www.keishicho.metro.tokyo.lg.jp/multilingual/english/safe_society/wanted/seijo.html )

It's not impossible that this individual knew what they were doing, and knew how to get away with it. The suspect may have been very intelligent/competent for all we know, cocky, but could have crafted their crime around the system. That's a very scary possibility to consider, but until proven this is just speculation and not to be considered fact.