Two horrible things happened. First, an innocent child was slaughtered. I hope that his family and all Yidden shall be consoled. And his neshama won't linger much before entering the Gan Eiden. Whatever tikkun it was supposed to accomplish in this world, it surely has done so. And something good will come into this world as a result of this tragedy, not just hidden and wrapped in mystery, but also in a revealed way.
But there is another side to this story. A person whose soul is literally a piece of G-dliness above committed a terrible, horrible crime. It doesn't matter if he is mentally ill, or if he did it deliberately. Whatever it was, it was his out "garments" that lead him to the murder. And buried deep inside him, there is that spark of Holiness, plunged into a dark, painful exile, screaming out in horror every second at what he has done.
(Read the "Note" at the end here.)
We must cry out and pray to Hashem and complain that just like this act was senseless and evil, this exile is also senseless and evil, and it must end. Not just because we are suffering, spiritually and physically. But because Hashem's Presence is in exile and darkness. And this is not right. It must not continue.
Everyone that feels empathy to Leiby's parents and the horror of this situation must realize that the global situation in which we have been living since the start of this final golus two thousand years ago is equally or even more horrible. And no matter how comfortable we are, physically or spiritually, no matter how many yeshivos we build, or how safe we are, we cannot be consoled until Beis HaMikdosh is here, Moshiach Tzidkeinu is revealed, and the world is full of knowledge of G-d as the oceans are full of water.