Wow, I was kinda surprised that people think Agasa did it. That has never even occurred to me. I mean, Agasa's such a sweet guy. Although that is why I am considered very naive...
As of now, I think it's someone we don't know.
And even if it IS someone we know, I'm surprised Heiji even came up. I'm biased because he's my favorite character but I just don't see him as evil at ALL.
As for Yusaku, I don't see him going to murder at all...
Okay, I'll admit it, I just don't want to believe any of the characters are evil. I love them WAY too much to believe that...