American Occupation in South Korea: Outline

If I don't post this, I'll never post anything.
With North Korea's recent threats to destroy everyone (again) and America getting ready to help South Korea defend itself if necessary, I don't know what to say. But I do question why we are still in South Korea, and if South Korea even needs (let alone wants) us there with them.

Back in December, a video of Psy performing an EXTREMELY explicit, anti-American song went viral. This was Psy participating in a 2004 concert protesting American occupation in South Korea. In the video, Psy refers to American soldiers as '--- Yankees' who should 'die slowly and painfully' with their families for interfering with South Korea's politics.
As an American, this really surprised me, because I always viewed American occupation in South Korea as vital to its safety against the war-ready North Korea. I thought that without American troops ready to fight, North Korea could easily bomb South Korea and conquer it again.

1. Opinions on American soldiers.
2. Opportunity costs
3. North Korea a real threat?