For Aquinas enemyhood is an accidental appendage, not a natural essence.

Evidence

In the first of these interpretations, loving the enemy as enemy, inquantum sunt inimici, he is already introducing an inner distinction (plausibly a distinction between essence and accident) that will be buttressed later on. Although Aquinas never directly defines the attributes of enmity, in this distinction, the being of the enemy is or not coextensive with his being a human being, with his nature. “Enemyness” can be contingent, the enemy may also be other than enemy, and therefore loving the enemy as enemy, insofar as he is enemy, is, again, perverse. Moreover, Aquinas explains, such love would have nothing to do with caritas, but would fall short of and even be opposed to caritas “because it means loving evil in another.” Such would be the quality of the enemy, then: It is what is evil in the other. Being the enemy is therefore not natural, nor does it signal an essential difference between the one who loves and the enemy. Rather, it appears to be an appended part, an evil part added to one’s nature. — Gil Anidjar, The Jew, the Arab, 180

Categorization

• DEEPTHINKING: The core conceptual move of the chapter: enmity as an ‘evil part added to one’s nature’ — an ontology of the enemy as contingency rather than essence. This is the load-bearing idea the whole book pivots on.