This link should help you counter any fundie arguments:
You can't really "prove" your point to them, as they will find some reason to discount whatever argument you make, regardless of how accurate it is. I would say the key is not to win the game, but to play well. The person you are arguing with will probably never concede, but a well-reasoned argument and a cool temper could make an impression on any silent observers who are perhaps more on the fence.
On the side, "thou shalt not kill" is from the notoriously poorly-translated King James version of the Bible, while more modern Bibles use the more accurate translation "thou shalt not murder." Killing is presumably acceptable if done in a lawful manner, such as sentencing a homosexual to death as ordered by God. In most cases, the Bible is only contradictory if you try to interpret it to mean something civilized. Taken as blatant bronze-age barbarism, most of it makes sense.