In its true biblical sense "an eye for an eye" does not prescribe or order extreme retribution. On the contrary, it means to limit retribution to no more than what was taken, in other words, losing an eye does not warrant taking two eyes, or an arm and a leg, or, well you get it
SO, in the sense it's meant "an eye for an eye" is related to aikido in that it prescribes reactions that are proportionate, and thus more ethical, to the provocation.
Knowing that you can't come out of a fight better than you went in, the best "win" is without fighting, and without damage. That not being possible, when an attacker means to cause serious harm or kill, and a quick solution is vital to preserve life, what about taking out a knee with a kick to disable, or an eye - and, actually, aren't those reponses "under"-proportionate to the threat involved? Sometimes love for who you are protecting, plus love for the attacker may not be attainable.