@UnderworldBlues
In his defense, two things (I'll state first off that Japan isn't America or the EU or anywhere else, different places of different laws around rights to remain silent (both in theory and actual practice), whether inferences can be drawn from that and when, etc. America's 5A is fairly strong).
But First, even under the 5A there is a reason that posts and videos have to constantly be made about "never talk to LEO without a lawyer". Exactly how the law lines up with common sense ideas of morality just isn't intuitive to most people. That they can lie, that many can be outright crooked even, still cuts against what the typical middle class law abider instincts and education. It's arguable that he should have researched this right away, but still it's a mistake a wide range of far more powerful people even make all the time.
Second and more importantly, at the end of the day he didn't really intend to "get away with it". He's not some hardened criminal, totally the opposite in fact, and under other circumstances he might have gone his whole life without ever breaking the law once. He just didn't want his family hurt, and a lot of his motivation for hiding it was specifically to prevent retaliation. But if that's off the table because the criminals are dealt with by the law instead, and he'd gotten his affairs in order, he'd just turn himself in anyway. That's not a "dumbass" thing. If anything the only "dumb" thing,
kind of, was even trying to cover it up in the first place vs reporting it right away and claiming self-defense. The circumstances we pretty legit, he was in his daughter's apartment, a violent drunk younger stronger murderer who was just talking about murdering his daughter and had killed multiple other women was literally threatening to kill him right that moment. In japan in might be harder, but at least in the states government might flat out decline to prosecute. And if it high chance of jury acquittal, or at worst some very low sentence.
Of course he was in between a rock and a hard place because under those circumstances a powerful criminal group might have sought retaliation extra-legally. So hard to call even that stupid. It was just a bad place to be.