It's just language. It has been pointed out that the notion of enemy always connotes at least the potential for killing him as a viable and legitimate way of resolving disputes. Even if we don't follow this analysis all the way, it should give us occasion for some serious thinking. Enemies are expendable. Enemies have no personhood.
Enemies can die.
When we see someone, or some group, as the enemy, war is the only appropriate means of confronting them. Of course from the Christian perspective, there is the injunction to love our enemies. We have to find ways to overcome the enmity. I am not sure that that is what we actually want. We are too comfortable in our opposing camps, oblivious of the dangers that our antipathies engender.