First, I would argue that in Matthew 5, Jesus's use of the word "enemies" really means enemies. It doesn't mean "jerks" or "grouchy neighbors." It means people who will seek to harm or kill you. In Matthew 5 we see early on that Jesus says we are blessed if we are persecuted for righteousness. While some take this to mean that only religious persecution is to be tolerated for the name of Christ, I would argue that Jesus is making the same distinction here as Peter does in I Peter. Peter acknowledges that there are at least two types of persecution - one for doing evil and one for doing good. Christians are not to be persecuted for doing evil. They are not blessed for that. Such a persecution is just. But if a Christian is doing good and is persecuted for an inappropriate reason, for this they will be blessed. While persecution is talked about as a result from our righteousness in early Matthew 5, that notion isn't necessarily religious, as I Peter helps us distinguish. Also, we later see Jesus talk about our persecution and love for others without any reference to righteous reasons or religious persecution. Christ's reference to neighbors, your own people (vs. an implied all others), the example of the tax collectors, and the like all point to a general understanding of loving everyone, even those who persecute you for reasons that aren't explicitly religious.
While Matthew 5 seems to allow for an interpretation of non-religious persecution, I would also again point you back to the early church. What were they teaching? What actions did they embrace? Would they buy the argument that they were free to kill someone in their community who attacked them? If they killed, wouldn't the community already know or find out that the one who took another's life is a Christian? Wouldn't the message of the gospel be colored by this action? Is there really a situation where we can guarantee that our Christian faith and therefore the gospel message can't be seen and then linked to our action of killing? If the church taught that the government bore the sword for punishment, wouldn't bearing the sword yourself, a citizen, in preemptive punishment, go counter to this notion? If some in the early church were speaking out against Christians even instituting capital punishment - a just reward handed down from a judicial system for a crime - do you think those same people would approve of the death sentence at the hands of a citizen, prior to a judicial judgment? People were allowed to defend themselves back in NT times, yet the sword was given to the government to bear.