If reasonable people are disagreeing, then they have reasons to disagree. If reasons can drive a discussion, they can decide issues like this. So your question is misleading. It's not "who decides", but "what decides", and how to we ensure the "what" remains reasons that reasonable people would support.

Otherwise, you're just asking how people make decisions. "Who decided murder should be illegal" is just a poor (rather conspiratorial) way to ask what reasons are used to argue that it should be illegal and how we determined that those reasons were sufficient.

Presumably, in a democracy, the answer to "who determines what the government does" is "the people."