While North Korean leader Kim Jong Il is a madman, and while North Korea was the first to kill anyone in today's skirmish, it was actually the South Koreans who fired first.
As AP notes:
The skirmish began when Pyongyang [i.e. North Korea] warned the South to halt military drills in the area, according to South Korean officials. When Seoul [i.e. South Korea] refused and began firing artillery into disputed waters, albeit away from the North Korean shore, the North retaliated by bombarding the small island of Yeonpyeong, which houses South Korean military installations....
And see this: ”S.Korea says it test-fired in area before N.Korea firing” (Reuters).
In addition, the two South Koreans killed were marines, not civilians, stationed in a military town.