Because you are missing easy points.
An adult have a right to defend himself even with heavy weaponry. But if he kills someone (not by self-defense) he is a criminal who choose it, so he can be killed of.
A baby didn't chose anything and shouldn't be killed because parents decided they can't bother.
What we're talking about isn't a baby. It's a bunch of cells that don't even have a heart beat let alone a heart. I'm pretty sure it's not murder if you kill something that isn't even alive.
If you demand respect or gratitude for your volunteer work, you're doing volunteering wrong.