Because your presented argument:
P1: God tells us we ought not murder
Con: Therefore we ought not to murder
Is a non-sequitur, the conclusion does not logically follow from the premises, because there is another premise that is hidden in there, that you ignored, as I already explained this
P1: God tells us we ought not murder
P2: We ought to listen to god
Con: Therefore we ought not to murder
perhaps your confused because the word "ought" is in the first premise. Please read your own lines carefully and employ comprehension while doing so - it is describing an alleged fact of reality - that god's rules say not to murder - that is not an ought that is an is - a description of reality (allegedly).