Exactly. A fetus is not yet whole.
This is true but neither are many adults. My use of whole should only suggest that the fetus is growing and learning towards maturity.
Neither is a sperm. If you disagree, explain why you consider one whole, but not the other.
A sperm is not a separate human being but just a cell. A sperm in no sense of the word grows into a human being. Though it may contribute its genetic code to form a human being, it is neither a human being nor can be. A fetus, however, is growing towards becoming a full human being.
Thus, while we can say that the fetus has the potential to be a human being, before conception the fetus did not exist in any form. Before conception there is no single entity that we can say has the potential of being a human being but only the sperm and egg of which we can loosely say share the potential to become a human being. But since the human being does not currently exist, the potential is not applicable. For murder to occur there must be a human being, and there is no human being here.
As an example of potential, say I was to tell you that you were to discover a new, important, theory a week from now if your office is not broken into. This was told to me by an unknown source but you do not know. Now say I break into your office, steal your papers, and then derive the same theory from it, calling it all my own. True, this is hypothetical, but am I not responsible for stealing the theory from you? If you did not exist, then you would be nothing to steal from. But since you do exist, I have stolen from a human being.
Although potential is important, one of the most important things is that it often requires foreknowledge of the potential. Someone can unknowingly destroy potential while still remaining relatively blameless if the act is blameless.
Given that many old people are completely dependent on hospital staff and family, with very little productive "value" to the society, why do you believe we should keep the old alive?