I respect your question and have the same question from the reverse side. I similarly have difficulty understanding how Christians who follow a doctrine that appears to be so kind can behave in ways that seem so cruel and heartless to the rest of us.

I suspect it is down to unbridgeable axiomatic differences.

You see a fertilized egg as a human with rights, I see it as a cell with interesting potential, not particularly more deserving of rights than any other cell with potential.

Even if I accepted that a fertilized egg has human rights, I don't understand why those rights outweigh the autonomy of the donor/host. What if I told you that your blood, bone marrow, kidney, or even lung could save another human's life at relatively minimal risk to your survival? (It's true, and I recommend everyone reading this be an organ donor and sign up for bethematch.com) But should the state be allowed to legally compel you to undertake these risks to save another human life?

Judging the stories of pro-life advocates who get abortions while convincing themselves that "their situation is different" my assumption is that there is a basic lack of human empathy. I've found that this squares with the fact that many people oppose benefits and rights for others but then begin to support them when it personally impacts them.