They could have reported it to the police. A serial murderer in fugitive is likely to do anything. Surely it's easy to believe that they killed him in self-defence. Why waste 50 years of your life on that scum?
And how can blood splashes look like cherry blossom petals? That's stressing too much.
I think these kinds of moments are what makes Kindaichi really stand out. The human element they put into some of these cases helps make them memorable, even if they feel like they're forced in a little bit.
Though sometimes, life is actually mysterious like that.
