Is killing vermin and insects justifiable?
The beauty of the precepts is that they are impossible for ordinary human beings to fully observe.  The precept against killing is an example of this. My body's immune system kills all lifeforms that it doesn't recognize.  I step on and crush innumerable microorganisms.  But the first precept asks us to minimize the amount of harm we do in the world.  In the teachings of Thich Nhat Hanh, the precepts are trainings, not commandments.