Legalized killin', eh? This thread sparked a memory . . .
I wrote a draft about a post-apocalyptic society that developed a simple system of justice. In the absence of prisons, anyone convicted of a crime would be declared outlaw and banished from the realm for a specified period of time, and possibly required to make some form of restitution before reintegration in that realm's society.
If an outlaw were to illegally return to the realm during the period of banishment, any citizen of the realm could take the outlaw's head and claim a bounty commensurate with a sliding scale reward system. While not strongly encouraged, an outlaw could be hunted outside the realm during the period of banishment, but a head taken outside the realm would merit a proportionately reduced amount of reward. This society did not have the financial burden of a prison system. Instead, it developed a potentially lucrative and self-supporting industry of bounty-hunting, open to all who would chose such a career/lifestyle.
A hunter who was very good could become very wealthy. However, the big problem for the bounty hunter was rather significant. A bounty hunter was not allowed to make a mistake; take the wrong head--be declared an outlaw . . . Or even worse, be framed by a jealous rival with an agenda for taking the wrong head . . .