This teaching is clear when dealing with the killing, butchering and eating of animals. My question is how far down the animal chain of complexity does this teaching go.
Is it OK to kill mosquitoes when one considers that they kill large number of humans via malaria.
Is it OK to use flea powder on one's pets when the option is allowing one's pets to suffer and perhaps die?
Is it OK to take antibiotics to cure a disease when the purpose of the antibiotics is to kill bacteria?
One very important faculty that we must utilize is our "wisdom". The wisdom to see the bigger picture; to realize the greater good of performing certain unwholesome acts; to come to terms with this Dukkha of the world.
If everyone refused to kill even an ant, then who is going to do the "dirty jobs", so to say. Who is going to defend the country? Who is going to keep law and order in the streets? Who is going to get rid of the pests that may affect our health? These are all very urgent and realistic questions. Those who are in this category may take consolation that they are doing it for the greater good of the society and nation. However, at the same time they could strive to lead a dignified, noble and harmless life as best they could. They still can follow and practise the teachings of the Buddha to the best of their ability.
The realities of this world and this existence are such that there are always this unsatisfactoriness and imperfections where many a time we are confronted, without much choice. As for taking life, we must also consider factors like its life-span, its usefulness, its harmfulness to others and the circumstances. If the life has a very short life-span, is not useful to society, and causes harm to others, then there is some consolation and mitigation if one kills it. However we must bear in mind that the act of killing for whatever reason is an unwholesome act. We have to use our wisdom, common sense and intellectual judgement to draw the line. It is up to the individual to set his "standard" of practice.
Here are my counter questions to your 3 questions:
Is it Ok for humans to die by refusing to kill the mosquitoes?
"Is it OK to use flea powder on one's pets when the option is allowing one's pets to suffer and perhaps die?"
Is it OK to allow your pets to suffer and die by refusing to use flea powder?
"Is it OK to take antibiotics to cure a disease when the purpose of the antibiotics is to kill bacteria?"
Is it OK not to cure a disease by refusing to administer antibiotics? (Actually bacteria are merely living organisms without consciousness)
The choice is yours. We must also bear in mind that we can take certain precautions to ensure that these problems be minimized with proper hygiene and healthy living.