NK's leadership is obviously bloodthirsty. Sooner rather than later, they will attack. Also, they are leftists - and leftists kill people. Take their famine, for instance. The leftists also make concentration death camps.
How should the world preserve the peace and save lives, do you think?

I think we should do so by a preliminary disarmament, followed by liberation by education.
Surgical air strikes, complemented with limited aeromobile operations to make sure all of the leaders are indeed dead, would be a good start. Several more strikes to make sure there is no peace-violating action from the leftist army on the South Korea borders. Eliminating majority of the officer corps and leftist leadership would open the possibility of re-educating the people, to prevent deaths by famines and other leftists ways to murder (concentration camps, political processes...)