The issue, of course, is what religion are they espousing. Biblical Christianity places a very heavy burden on such a misuse of God's commandments. That is an impossibility. Furthermore, the person who argues that the command against killing forbids all executions is implicitly equating murder and execution. Both acts are viewed as being evil and essentially the same thing.
Murder is the killing of a man by an individual and execution is the killing of a man by the state.
However, to oppose executions for reasons of evangelism poses several problems. First, the Savior of souls is the Lord, and His will is not dependent upon man. While it is the church's duty to preach the Gospel, it is God who will decide if or when a person is converted. Civil government's role is to punish evil, not to delay justice for what may or may not eventually turn out to be a genuine conversion. This would paralyze the legal system.