I personally do not start at the point that a human being cannot be killed and then ask whether a foetus is a human being or not. Any answer to the question of whether something is a human being is a value judgement. It is circular reasoning (we shouldn't kill human being because they are human beings).
More we should ask... Why don't we terminate life. Because there is generally no negative consequence to keeping the person alive. Because the person chooses, or would choose if they were in a different situation, to live. Because the process of death is unpleasant to them. Because it would cause devastation to their loved ones.
None of this applies to a foetus and thus it's 'death' is fundamentally different from killing that is illegal.