Great. So, let's run with that.
Anyone who commits a felony has demonstrated that they could not, at that time, live within the laws of society. Therefore, they may at some future time also not be able to live within the laws of society. Therefore, the only reasonable courses of action for anyone who ever commits a felony is life in prison or execution - because you never know.
Also, we must assume the system is perfect - nobody is ever unjustly convicted, and has those convictions upheld by numerous courts of appeal.
Of course, there is also the real world, wherein unjust convictions have occurred, and been upheld in the appeals process.http://www.innocenceproject.org/
And there are some felony convictions which shouldn't result in lifelong imprisonment or execution - thus our system of parole, probation, and other systems of treating individuals as individuals.