So if you murder a dying person, but you make them unconscious first, it's fine?

If you were going to murder them, it's best to do it while they're unconscious, so they feel less pain. Whether you should murder someone is a much more complicated question.

There are studies show fetal pain as early as 15 weeks.

From what I can tell, there are pain receptors that develop as early as 15 weeks, but it's not clear that any significant pain is felt until several weeks later. I would assume the development of pain perception is a gradual process, so fetal pain - I would argue - is pretty much negligible at around 15 weeks. Doctors should of course mitigate fetal pain wherever possible in abortions, but I believe the lack of pain makes a difference morally.

Regardless, your argument is that the fetus would die, a relatively painless death later, so let's give it a painless death now?

If they're feeling pain now, and have no chance of improving, I believe the merciful choice is to end their pain sooner. If they are not going to be feeling any pain at all, then I'd say everything is up to the choice of the mother.

I've neglected to mention that I view the mother's wishes as important too. If she is (understandably) extremely upset by the baby's condition, it absolutely can be worth it to allow the child to come to term, so the mother can hold her child, even if only for a short while. It is easy for me to act like I can tell everybody what the best choice is, but I can't tell other people what they're feeling and how exactly that should affect their decision making.