Why is a single article such as "whoever kills another shall be punished with ten to fifteen years" said to contain two norms rather than one?

D. It addresses the citizen with a prohibition and the judge with a duty to punish as distinct addressees.

Because two different people are being addressed. The primary norm tells the citizen not to kill and is nowhere written; the secondary norm tells the judge what to do if someone does, and is the only part that appears in the text. The criminal law reaches the citizen through a sentence addressed to somebody else, which is a genuinely odd feature of the field.