The main question that comes to mind reading this thread for me, and it seems like a rather important one for Dhamma practice in general (not just this specific case), is this: can kamma be made through not taking action? If it is within one's power to change a situation, and one does not, does that create kamma? To me, it seems most likely that it would, but I would be very interested to know what those better versed in the teachings than I am would say about it- I can't recall any direct statements on this question, but I think it's implied that it does- the kammic rewards for keeping the precepts are spoken of in the suttas, for example, and the precepts are mostly matters of restraint rather than active action. If that's the case, it seems logical that inaction in certain situations could create bad kamma as well- and in that case, letting a living being die in a horrible way when one had the power to prevent that outcome would seem to me to be a pretty clear-cut case of something which would create bad kamma.
I have to say that I find Kevin's position in this thread impossible to stomach, and I don't think it's just because of attachment to cute animals or the like. If it's really the case that the proper Buddhist thing to do in this situation is to sit back and allow an animal (or a human- it seems to me that the logic applies just as much in both cases) to die in agony, when one is perfectly capable of preventing this from happening, all I can say is that I want no part of any religion that advocates that- I would say exactly what tiltbillings did, above, that I'd walk away from Buddhism and never look back if this is actually what it teaches.
I don't believe it does, though, so far as I understand it- assuming all the premises of Theravada Buddhism are correct, I don't at all deny that some degree of dark kamma would be made by killing ticks and fleas- but what I find extremely difficult to believe or accept is that dark kamma wouldn't also be made by sitting back and letting a living being die horribly when one had the power to change that outcome. Kamma that is "dark and bright with dark and bright results" is specifically referred to in the suttas- a black-and-white approach seems to me to be not only not true to the realities of life, but not true to what the suttas say, either. Killing fleas to save an animal would seem to me to be pretty obviously "dark and bright", and which one was the greater would depend on lots of things- however, I would guess that if it were done based on compassion towards the animal, it would create significantly less bad kamma than letting an animal die in agony because one feared the effects of bad kamma would. All this implies that there's essentially no way of avoiding making bad kamma in this situation- but I think life contains many situations where it's not really possible to avoid it, and that this is part of the reason why we're trying to get out of samsara in the first place.
(And yes, an arahant wouldn't kill, but one thing that puts a very different light on this entire discussion when it comes to the question of what action an arahant would take is the fact that, according to all the traditional teachings, siddhis exist. And, as I understand the teaching, they are most easily learned by Noble Ones. Needless to say, this would give an arahant ways of handling situations like this which are not available to the rest of us. I don't believe that an arahant would kill the fleas, but I also don't believe that an arahant would just sit back and watch a living being die in agony when they had the power to prevent it- and this is where the siddhis would come in, most likely. The existence of siddhis is another debate, but if one accepts kamma and rebirth, they hardly seem like much of a leap, and if one has faith in the teaching of the Pali Canon as a whole, they're pretty much a given.)