What's their motivation? Why did they attack? Why is NK not an ally of Western nations, US etc?
According to the LA Times:
"The North Korean government is trying to strengthen internal unity and solidify Kim Jong Un's succession by creating tension among its people," said Kim Yong-hyun, a professor of North Korean studies at Dongguk University in Seoul.
"Pyongyang is using shock treatment," said Kim. "The North is sending a clear message to the international community and especially the United States that there remains tension on the Korean peninsula."
"It's pure brinksmanship against the U.S. and South Korean governments," said Lee Dong-bok, who is also a former negotiator with Pyongyang.
"I see this as a last-ditch effort to engage Washington and Seoul. They want to return to negotiations but they want to do it on North Korean terms."
This is what i see as the major cause:
North Korea didn't get the desired effect when it made its nuclear facility public. South Korea didn't back down from its hard-line policy," he said. "On the contrary, Seoul and Washington have in the past come out strong on the North's provocative actions. Pyongyang wants to stir up tensions on the Korean peninsula."