To be a Christian is to be persecuted. Paul is dealing with what our reaction should be; not just a neutral reaction but blessing the persecutors. This is the teaching of Christ in the sermon on the mount, and His example on the cross. This is not the ordinary response in a sinful world but we are not to be conformed to the thinking of the world. We desire for divine triumph and justice over sin to be seen, but do not desire personal vengeance. If we call down a curse, or even harbour a grudge against a brother or sister, we do not understand the grace of the gospel.