Killing some other living thing, under almost all our ethical system is considered very bad, and wrong. Generally the more aware and intelligent we consider the living thing to be, the more wrong that killing may be.
So when is killing justified? Commonly when the needs of humans are in question, the killing is justified. A lot of times when the desire of humans are expressed, killing is also justified. We as humans and especially in the 90's can see a lot of examples of this in everything from our diets to our clothes, perfumes to medicines. We as humans are in fact, breeding species of animals, plants microbes, and almost all other living things in order to kill them. So I believe we are doing harm to ourselves. But still the question then becomes, are our purposes enough justification for our actions? Then again what are our purposes, what are our actions, and what are our options? I myself believe that in the case of rape and incest, only the most fervent opponents of abortion will say that it is unethical. Where a mother's mere whim is at stake and a partially developed child on hand, only the most fervent supporters of abortion agree that is ethical. I think that from that we can see that the majority of people are in an abortion - middle ground, yes it does exist, but how to define it is the nest step. .
I think that most of us believe that life is considered to be very precious for a lot of reasons. It is because all of us are very intelligent, civilized, agricultural, and also several other attributes that we are different from animals in a scientific way. Some people say that because we communicate very well, some say that it is becaus4e we are very creative, and because we use tools. But regardless of your own definition, there are none exceptional humans as well as exceptional monkeys that blur these distinctions. We as humans need to decide for ourselves when and where is a human valuable or not valuable.