Can Killing Sometimes Be Justified?

I’m something of an expert on bed bugs – having lived in India for a few years. Generally I like all creatures but there three that I just can’t abide, at least in my private space – flies on my food, cockroaches in my residence and bed bugs in my bed. Years ago, before I was a monk, I was in Kashmir going from Jammu to Srinagar and had to spend a night in a truck-drives’ stop. This insalubrious establishment consisted of one large windowless room with an earthen floor and filthy quilts and blankets scattered around the place. My companion and I selected the least filthy of these quilts, found the least dirty part of the floor (near the wall), blew out the lamp and huddling together against the cold, settled down to try to get some sleep. About half an hour after the light went out we both began to itch, first just a little then a lot. Grumbling and sighing, we lit the lamp again and found dozens of bed bugs on our clothing. Then a look of horror came over the face of my companion and he pointed towards the wall. I looked to where he was pointing and there, marching down the wall, were hundreds of thousands of bed bugs. There were so many of them they literally formed a rusty-colored curtain on the wall. Believe me when I say, ‘I hate bed bugs’.
From one perspective Cimex lectularius is a fascinating little creature. They are mentioned in the Tipitaka (e.g. Ja.1,10; III,423) where they are called mankuna. They can live for up to a year without feeding. Although slow-moving, their repulsive taste protects them from predators and even we humans are reluctant to squash them because of the offensive smell they give off. They even have a political dimension. By the early 60s they’d been almost eliminated in the UK. Then Mrs. Thatcher got elected and within three or four years they had returned with a vengeance. If bed bugs could vote they would always support the candidates advocating ‘trickle-down economics’ and promising to cut welfare payments and ‘streamline’ the health services. The other thing you have to admire them for is their tenacity. Once you get bed bugs they are extremely difficult to get rid of.
Well, a fortnight ago I found a bed bug in my bed. I was not a happy monk. The next morning I thoroughly checked the mattress, sheets, pillows, the bed itself but found none of the tell-tale signs of their presence. A week later another bug crawled out of a book I was reading. Then a day later I found another one. I realized it was time to take action. I took all the furniture outside and tipped boiling water over it – not just to kill the little blighters but also any of their eggs that might be there. Then I went out and bought some insect spray and sprayed every nook and cranny in the room. I soaked all my bed sheets, pillowslips and robes in boiling water and then put them through the washing machine twice. Then I put all my books in large plastic bags, sprinkled them with moth balls (naphthalene) and sealed the bags for a week. Now I’m waiting. I’ll keep you updated.
But while waiting I am also thinking. I know I deliberately and with full consciousness killed three living beings and probably killed at least a few more with my boiling water and insect spray. I’m a Buddhist monk, supposedly a model for how the devote Buddhist should live. But are their circumstances when killing, at least killing insects, is justifiable, or perhaps understandable, or perhaps excusable?