Let's get the truth clear here: South Korea were carrying out military exercises using live arms into disputed territorial waters. The 'line' which has been drawn by South Korea is not accepted by North Korea, and South Korea were firing missiles onto this 'line' in the sea.
I think most countries would see this as provocation, and North Korea fired back, into the disputed waters. South Korea fired back into North Korean territory, i.e. land, and North Korea retaliated by firing back. The USA has now sent a warship to the area, which of course raises the political tensions even higher.