I'd say yes, but it depends. I won't say "no - unless" because that's in effect a "yes...if". Self-defense doesn't really echo with me much, cos if someone else's life is expendable (which I must have concluded in order to kill them), then so is mine. But if instinct were to take over, I'm sure I'd ignore that and kill anyway.
However, experience and intuition tell me that a) there's always another way, and b) "the other thing" is never as bad as you think it's gonna be and c) often we fight hard to preserve something under the assumption that whatever replaces is won't be as good, but this assumption is often false.
Whatever happens, one can always adapt. So I can't see any situation as being so fantastic and important, or any prospective situation being so intolerable and unworkable, as to be worth killing to defend/avoid.
And I guess I'm working under the assumption that killing is a last resort, because of what I said before: to admit someone else's life is expendable is to call my own so, too.
However, "those who cling to life will lose it"... I have met people who seem to cling so hard to their life and youth and stuff that they become enveloped in a cloud of 'decay'. If I were to cling so hard to my life as to end another in defense of it, I think the life I'd have after that wouldn't really be a full life, so I would hope that, when it comes to the crunch, I'd be as ready to let go as I am ...well, when it's not crunch time!