Have we really gone thus far without acknowledging the pun?
I haven't found any concept of physical self-defense in Orthodoxy - only (1) spiritual (i.e. against the passions) and (2) defense of others. There are loads of references (scriptural and patristic) about loving our enemies, laying down our life in love, etc. when it comes to threats against our own persons.
Sanctity in an individual is not an endorsement of all their life's actions (and we see this play out with all the Saints). And yes, there is a canonical prohibition against a priest taking a life, even accidentally. Like all canons it can be waived by the person's bishop, but that is relatively rare (outside of the Serbian Church).
I think you're fundamentally misunderstanding the Orthodox concept of sin. Anything short of perfection is sin in our eyes - even if only short by a small degree. So yes, taking anyone's life for any reason, even if generally considered upright (i.e. in defense of the innocent and helpless) is still sinful (i.e. less than perfect) in that it isn't exactly what Christ did / would have done (which was to sacrifice Himself in order to save us).
And (as supported both in our historical witness and in contemporary psychology) regardless of intent or justification, taking a life wounds a person. In this we see the true nature of sin - our missing the mark changes us, makes us less like our prototype, and leads to (varying degrees of) disorder and dysfunction. Thus, penance is less about atoning for sin and more about healing: healing for the individual who sinned, healing for the one sinned against, and healing for the community affected by sin. Even separation from Holy Communion is not done as a punitive measure, but as a wake-up call and protective measure for both parties, earnestly desiring the repentance of the sinner and the acceptance of the community through the absence (which isn't supposed to be an absence - you're supposed to come to Church even when you cannot receive communion, and use this as a chance to repent and to receive forgiveness).
Yes, a distinction is made between killing in self-defense versus killing in aggression, but that distinction doesn't mean all don't receive or deserve penances. All wounds (accidental, self-inflicted, and inflicted by others) need healing, and we take that principle and apply it to wounds against the soul as well.
