The ends never justify the means. According to the Catholic Church, it is a sin to directly kill innocent human life, no matter what advances science or medicine could gain from the remains left after the innocent life was unjustly taken.
Some ask, “What about the already aborted embryos which are dead anyway? Why not use their bodies the way medicine uses corpses today for organ donation?”
The method by which the embryonic stem cells are obtained is still sinful and immoral whenever it involves the destruction of a human embryo since that would be the unjust death of an innocent person. If an immoral government unjustly takes your property and offers to sell it to me at a very cheap price, I cannot in conscience buy the stolen property no matter what good use I could make of it.
Likewise, murdered embryos are still victims of direct and unjust killing. Furthermore, though potentially lucrative for some pharmaceutical companies, the prospect of embryonic stem cells is neither foolproof nor risk free.
There is credible research which shows that adult stem cells and umbilical cord stem cells are just as viable as embryonic stem cells, if not more so.