But, jmf, the authorities referred to in all your quotes are non Christian!
Most importantly, by claiming these "agents of God" who execute wrongdoers can not be Christians because the death penalty is sinful, you're basically saying that God has authorized and commissioned non-Christians to commit sin.
Obviously, such logic is flawed and contrary to the rest of scripture.
I was viewing this more from a historical, rather than logical, perspective. Christians, at the time these letters were written, were living in midst of a nonchristian society. Obeying its laws, which included the Deaht Penalty, strikes me as merely a sane course.
When Jesus had the power to prevent an execution, to be committed by his own people, he did so, at great personal risk to himself (that woman who was supposed to be stoned).