when it becomes a human is the debate issue here
Because of the controversial nature of the debate, using precise terminology is very important. Because fertilized human eggs are genetically human (species Homo sapiens), they are all 'human'. The correct term to use for this debate is 'person', not 'human'. So, I would rephrase your statement as:
when it becomes a person is the debate issue here
For the anti-abortionists, the reason 'human' is not the correct term is because every cell of a human's body is 'human'. You kill far more human cells when you scratch your nose than when a woman gets an early abortion. But your nose skin cells are not persons. They have no minds, no feelings of their own, etc. You're not committing mass murder when you scratch your nose. (Now at least you can get to sleep tonight.)