It is the hard way of revealing one *almost* perfect crime. We heard often about revenge best served cold, well then, this was 'killing best served at the fresh time given', even if you prepared it a long loooong time before hand.
I love Higashino-sensei works. He always give twists that surprised me. With this one, he played with 'how' this murder was done. And of course, it had to be THAT simple yet, ilogical impressing way. Superb!
It was an amazing story