Should governments behave in a Christian way?

I was in a debate with someone recently who claimed that "God does not require governments to follow Christian behaviour", even if those governments are composed of Christians, are elected by Christians, and explicitly call themselves Christian. The claim is that this means it's OK for Christians to vote for a government that violates Christian behaviour on a regular basis. I'm interested in arguments *against* this position, and in order not to make this question an argument let's restrict ourselves to reasons that governments *should* (in general) follow Christian teaching. I'm aware that there will be some things that governments are permitted to do that a private individual shouldn't, like enforce the law or go to war, but I'm not talking about those. I'm interested in countering the idea that governments may do anything they like, no matter how much it contradicts Christian teaching. Answers from a Protestant viewpoint preferred, especially Evangelical Protestant.