Blastocysts are not people. Individuals who suffer from debilitating diseases are, however, and, if killing thousands of human embryos provides a cure for something like blindness or diabetes, it is not only ethical but morally obligatory to do so. It's really that simple.
Only if it's being done to save and improve lives. Umbilical cord cell research works just fine. I believe that life should not be created to be destroyed. However if it's between stem cell research and no stem cell research but people dying of illnesses and disabled than it's ok