When does it become a human being? Is it justified to kill the clearly human organism before it becomes a human being?
It becomes a human being the moment it becomes a distinct individual. Two parents become two parents and a child during fertilizatoin. Identical wins go from being one individual to being two individuals when the zygote splits. Chimeras go from being two individuals to being one individual when they lose a distinct individual identity. This works well even with the hard cases to define and strange science fiction examples, such as a Star Trek transporter splitting one person into two people.
The key is understanding that "when does a human's life begin" is a trick question. The cells involved are never dead because dead matter does not spontaneously spring to live. That quaint idea went out with the development of microscopes and the discovery of bacteria, sperm, and eggs in the 19th century. The question isn't "When does the baby become alive?" because it's never dead. It's also never not human at any point. So the question here is, "When does it become a distinct individual?" which is what's generally meant by "human being" and the answer is typically at fertilization, which is the problem here.
Now, if you want to try to draw a line somewhere between that zygote and an adult human where the human being in question develops some set of traits that you want to use to distinguish between what philosophers genererally refer to as a "person" and a non-"person", you are free to do so, but an honest assessment of any such characteristics will quickly lead one the conclusions reached by Michael Tooley and Peter Singer, which is that to accept abortion is to accept infanticide because there is no relevant difference between a child before or after birth nor at any particular phase before birth after fertilization. If you want to test that, ask yourself if finding the same characteristic in an animal or alien lifeform would be sufficient to consider it a person. And before considering the mental capacity of a newborn to be an obvoius distinguishing characteristic, please look into the true non-romanticized mental capacity of a newborn really is.