When someone has a right not to be killed, that doesn’t mean simply that it would be better for someone not to be killed. It also doesn’t mean simply that others ought to avoid killing someone when and where such avoidance would be possible or convenient. What is meant by saying someone has a right not to be killed is that others *must not* kill them – that is, someone with a right not to be killed has a *demand* against others such that they are obligated not to kill her. This demand has a force that goes beyond any sort of idea that, all things considered, it would be better for someone to live than to be killed.
When someone has a right not to be killed then killing them can be excused or justified only when there are clear, compelling, convincing and relevant reasons for why killing them is the only acceptable course of action (and such cases must be rare). Rights have another function, and that is to treat as equals *all* those who have the same right. So, if humans and microscopic animals all have the right not to be killed by others, then humans and microscopic animals have that right in the same way and to the same degree. That means that in cases where microscopic animals will be killed by human actions, human beings who kill microscopic animals will be as guilty of wrongdoing as would be a man who kills his wife. It also means that, just as we would sanction the use of lethal force if necessary to prevent a man from killing his wife, we would have to sanction the use of lethal force when necessary to prevent the killing of microscopic animals.
I hope that anyone reading this can see that it is not possible for us to treat microscopic animals as if they have a right not to be killed – it would be absurd to think that people have the authority to kill other people simply because the actions of other people cause the deaths of microscopic animals. It is not possible to live as human beings without causing the deaths of microscopic animals. Therefore, it is not the case that *all* animals have rights.