Not only that, but if Jormag truly wants to manipulate people into following him, killing one of Tyria's greatest and most famous heroes would be counter productive. On the contrary, Jormag probably wants the Commander on his side more than anything. What better way to sell the idea that he is the "good guy" than getting the Pact Commander, the slayer of dragons, on your side. Sparing the Commander is probably just a means to an end, not a sign of benevolence.