I don't know much about stem cells and I know some but how is it killing a baby?

I know:that when a cell is reproducing (mitosis) the new cell that soesn't have the copied nuclei is a stem cell because it doesn't have the nuclei to tel it what to do...Now some people are against it, why, how does killing a baby come along? Your cells are constantly reproducing why is it bad to just take one? I want to know what the scientists are doing that is bad.