What Yoshito did just made everything more difficult for his parents. He lost the purpose for not wanting to make them sadder. I believe his parents would be more glad if he just came home that day rather than becoming a murderer. But well, he's a kid who just experienced something traumatic. I can't really judge him.
I'm not taking any sides as I think both of them are in the wrong. Still, I pity Yoshito for what happened to him and I don't like those 6 teenagers' cowardly and selfish action.