"After all, morality is about choices: murdering an innocent is presumed to be "wrong" because one could have had refrained from this action, yet chose not to."

No, what you have stated is not why murdering an innocent is considered wrong--what you have stated is why one is considered responsible for the act (i.e., that one could have chosen to refrain from the act but did not).


"If a crazy , humongous serial killer grabs my arm and uses it to beat a little old lady to death, most people would not say that I did anything wrong: while it's technically true that I beat Grandma to her grave."

No, it's not even technically true that you beat grandma. The serial killer did it using your arm, and if you either resisted or did not in any way consent to what the serial killer did with your arm, then it's not much different than the serial killer using a hammer to kill grandma (we wouldn't say that it's technically true that the hammer killed grandma).

Why do you call what you say a "refutation of determinism"? It doesn't seem to me that what you've said bears on the truth or falsehood of determinism at all.