"Murder is wrong" is one of the sublest cases of begging the question there is, because it's not "killing humans is wrong" -- the majority supports the death penalty, most people support defensive wars, and so on. What "murder is wrong" translates to is "we don't approve of killing humans in ways we don't approve of," which is tautological. The moral judgment comes in "what sorts of killing don't we approve of?". Generally, we can agree on "killing another adult human who did nothing to you, because you wanted to kill someone" being wrong. Beyond that, though, it's a mess (how about killing someone who abused you? who abused your children? who threatened to kill you? who was acting in a way that endangered you or your loved ones? who invaded your private space? ... and so on).