Stem cell research, as such, is not controversial or unethical. The basic objectives of such research are good—the development of medical treatments and cures. The controversy surrounding stem cell research relates to the sources of stem cells chosen. 

Specifically, embryonic stem cell (ESC) research is controversial because it necessarily involves the killing of human embryos.  Some who try to justify the destruction of human embryos argue that they are not human beings. Human embryos are, indeed, very tiny. But they are human beings nevertheless. You and I were once embryos like those killed to harvest stem cells. The scientific fact, confirmed by every embryology textbook, is that human embryos are human beings.

Other forms of stem cell research do not involve the destruction of embryos. As well as ensuring that the research is ethical, this avoids developing treatments which many patients would feel conscientiously bound to refuse.