First a disclaimer. I am not a professional ethicist, nor a moral theologian. I do not have a degree in theology. I have a BA in literature and an MS in nursing. I have been a Catholic since 1973, a nurse since 1985, and a nurse-midwife since 1995. I have studied ethics formally and informally, and have given presentations on midwifery and ethical dilemmas in several forums.
Most of us go through life without having to personally face serious ethical dilemmas. We are able to make more or less ordinary decisions using the tools of our knowledge, experience, and faith, with occasional consultations with various experts. The formal study of medical ethics is usually only brought into play when there is a dilemma - a difficult choice to be made.
Resolving the dilemma requires both a clear understanding of the facts and of the available choices. These are then balanced against moral principles. As Catholics, we believe in many absolute moral principles, and there are also some relative principles. A basic undergirding principle in Catholic moral theology is that one might not do evil in order to obtain (a) good. In other words, the ends do not justify the means. As an extreme example, it would not be moral to send a time traveler to forcibly abort Adolf Hitler from his mother's womb with the intended effect of preventing the Holocaust. One cannot require another to donate a kidney to save the life of another person (though this donation may be made voluntarily if it does not present an undue risk to the donor). One may choose to be generous and self-sacrificing - but this is not required. (Later I think we need to take a side trip into the difference between suicide and martydom, but not here, not now).
An absolute moral principle is the respect for human life (created in the image and likeness of God) from conception/inception to natural death. (I used the term conception/inception because there are those who do not believe that conception occurs until the fertilized egg implants - but inception is obviously the moment at which the two cells fuse and begin to divide and multiply). If one believes that a new life begins at that moment, when the two become one and that one begins to grow, then any deliberate interruption of the growth and development is gravely wrong. This is part of why the Church is opposed to ART (assisted reproductive techniques). When an egg and sperm unite, that is a unique and new creation. The name changes as that creation matures (zygote to embryo to fetus to infant to toddler to child to adolescent to adult to senescent) but the personhood of that creation remains intact. Certainly no one can argue that this creation, by whatever name one calls it, is fully human. (I will leave aside the even more complicated and as yet theoretical human/animal chimeras- I can give you the names of some SF novels to read if you want to explore that topic). Fully human, with a human mother and a human father.
Why embyonic stem cell 'research' is so ghastly wrong is that it intentionally sacrifices these humans at a very young age and 'harvests' their cells as fodder for potentially alleviating human suffering and/or curing human illness. There is nothing wrong with these goals (alleviation of suffering etc) but there is something freakily wrong and quasi cannibalistic about sacrificing intentionally created human beings who have been deemed surplus. Or maybe it is the whole concept of a class of human being being deemed surplus.
However, I do concede that there is a precedent or two that may have already numbed our moral sensibilities. I refer to the use of cell lines for vaccine research and manufacture that originally came from intentionally aborted babies. And there is the fact that we do make use of (sometimes life-saving use of) knowledge gained by the Nazis in experiments where they tortured subjects to death by exposing them to various insults. Here the dilemma is that the knowledge (or tools) were gained through immoral means yet the results might be life-saving. COG has a number of thought provoking articles on aborted fetal cell lines. You may choose to disagree with some of what they have to say (they have been accused by some of excessive scrupulosity) but I do think we need to at least think about moral culpability. In particular, Bishop Vasa (Baker OR diocese) has some well-reasoned arguments that invoke the 'ends do not justify the means' moral principles.
I will stop here for now. I obviously have a lot more to say but I will try to wait for your feedback.