I am not getting it!

I was reading the process used by the South Korean scientists for stem cell research. They take a female egg and replace its genetic material with the DNA from a patient, and prompt it to grow into an embryo. From that embryo they develop stem cells, which are a genetic match to their donors. This technique of creating the exact clone of ones cells has the potential of treatment for body transplants, spinal injuries and genetic diseases like diabetes, so on. According to some scientists, the stem cell research may even find cures for diseases like Parkinson’s and Alzheimer’s. Opponents to stem cell research appear to have two major concerns. One is that in creating stem cells one is destroying the ‘potential of human life’. I do not understand this. Do they want ALL female eggs to achieve its potential of becoming human beings? (Are they sad when a woman has her period??) The other concern appears to be that an “embryo is a person”, and hence one is killing a person when they make stem cells from it. This also does not make sense to me. The eggs are not ‘fertilized’ with male sperm to create embryos for stem cell research; hence cannot ever develop into a human being. Am I missing something here?