Well, this isn't only limited to Mexico though, in terms of the law.
My point, from the beginning, was that just because an illegal immigrant comitted a felony hit and run, stopping 100% of all illegal immigrants entering the states isn't going to stop all felony hit and runs (and I bet it statistically wouldn't even make a dent).
There are plenty of what-ifs here as well. The person made a terrible choice, but would he have run if he was granted legal citizenship? I am sure the risk of deportation played on his mind when he made that poor choice (not justifying, just musing).