I just read Ven S. Dhammika's struggle with bed bugs.
I think it's something like this. He killed 3 bed bugs. According to him, if he simply remove the bed bugs:
(1) they will bite someone else because bed bugs can only drink human's blood (at least the species he caught), and that means he inflicts other people, (2) if they are released somewhere far from human, they will die of hunger. (3) can't imagine how many other bed bugs hiding, waiting to feast on Bhante's blood if he didn't do the genocide immediately.
However, he certainly has killed with full intention, so he had a long discussions with people visiting his blog, and with himself.
So Can Killing Sometimes Be Justified?
http://sdhammika.blogspot.com/2010/03/on-2nd-march-i-related-my-bed-bug.htmlEDIT: In one of the related blogs, he said something about self-defense is not really wrong, or something like that, he had the reasons to support that.
Killing your ticks could fall on this category.