Why is an adversary an opponent or enemy? Etymologically,shouldn't the word mean "turn toward",giving the sense of an ally or friend? Consider the word attorney,which seems etymologically similar,and which has the sense of an ally and/or advocate.
Looking at etymonline, the two words seem indeed to have similar-meaning roots:
[..] or directly from Latin adversarius "an opponent, rival, enemy," noun use of adjective meaning "opposite, hostile, contrary," literally "turned toward one," from adversus "turned against, turned toward, fronting, facing," figuratively "hostile, adverse, unfavorable,"
one appointed by another to act in his place," from Old French atorné "(one) appointed," past participle of aturner "to decree, assign, appoint," from atorner "to assign," literally "to turn to"
But we can see clearly how two different interpretations evolved, and they did quite early on.
Turning towards someone can mean that you oppose them, you turn against them. The figurative interpretation is not difficult, if one imagines a battlefield: the ones turns towards you are not your allies..
Attorney, on the other hand, derives from a completely different interpretation of "turning to": an attorney turns to your side, is added to your "team".
It seems the original situation from where the turning takes place may help to explain: in a peace-situation, someone that turns towards you becomes your enemy, but if you are already being attacked, someone who turns to you(r side) becomes a friend.
The whole point about turning is that the effect depends on one's point of view...