The main reason why I oppose the death penalty is because I am not willing to be an executioner (I abhor the thought of killing another person), and I do not want others to do executions in my name, whether as a citizen of a jurisdiction that imposes the death penalty or even if I were to be murdered. It is true, as Mr. Prager says, that I am sympathetic to pacifism, but I do not hold to secular progressive thought's denial of free will. (I noticed that Mr. Prager did not discuss religious progressive thought in his column.)
Also, the Catechism of the Catholic Church says in paragraph 2267 that "[t]oday, in fact, as a consequence of the possibilities which the state has for effectively preventing crime, by rendering one who has committed an offense incapable of doing harm -- without definitively taking away from him the possibility of redeeming himself -- the cases in which the execution of the offender is an absolute necessity 'are very rare, if not practically non-existent'." Since I am a practicing Catholic, I think that this is a very important paragraph: your mileage, however, may vary.