The fact Levi Aron was a McDonalds-eating non-Orthodox makes a lot of sense.
Whatever he was, it is clear from the Police Commissioner and District Attorney that Aron was not a sexual predator, and had no criminal history at all in that area (or any area for that matter, other than some single unrelated misdemeanor.)
There was no way for any human being to predict what Aron was capable of doing (murder) beforehand, and even if there was there was and could be no legal mechanism to do anything about it or to commit him involuntarily before he committed any crime.
It is also sad that it turned out that the massive search for the child is what scared and caused Aron to panic and do what he did. It was entirely unpredictable, and the massive search was the right thing to do nonetheless, not being able to possibly predict such an outcome.