DtG: yes, the Old Testament does make it simple. The commandment says "Thou shalt not kill". This also applies to the original murderer.
Once the original killing has taken place then there needs to be some form of retribution, or some protection for society to prevent the same person re-offending. It is not until the appearance of Jesus in the New Testament that we see the idea of 'turning the other cheek'. In Old Testament times the simplistic notion was "an eye for an eye", or "a life for a life"
Don't forget, you're quoting the 'God of Sabbaoth' (the Lord of Hosts, or armies). This is a god who asked Abraham to sacrifice his only son, and only provided a 'sacrificial goat' as a replacement at the last moment.
Even in the New Testament, God allows His Son to be 'put to death', even after a prayer from Jesus that he be excused this fate.

I don't really think quoting the Bible is going to help either side of the argument.

CHEERS

Nigel