I can't answer thoroughly on how the policy of isolationism came about, although largely Kim Il Sung copied the brainwashing, cult of personality and propaganda techniques used in China and Russia (two of their communist brothers)

As for how defectors have reacted to South Korea, I've read quite a few testimonies - including the excellent 'Nothing to Envy' by Barbara Demick.

When North Korean's arrive in South Korea they are given a re-education for several months, designed to de-program them, bring them up to speed on world events they may have missed and help them cope with new technologies, freedoms and experiences they are unaware of. They also receive a government subsidy to help them buy an apartment and so on.

Some North Korean's are victims of prejudice, their malnourished state makes them stand out. South Koreans may regard them as inferior, untrustworthy, weak and so on. Naturally they receive a lot of attention and criticism for the regime they left behind, which interestingly many of them will defend against what they must feel is an onslaught of negativity against the culture in which they grew up, despite of course choosing to escape from it. Many refugees feel that the transition is difficult and might feel caught between the two worlds, one of which they cannot return to, and another they cannot assimilate into.

Another huge factor is language. The North Korean dialect sounds incredibly archaic, as language has altered so little in North Korea over the last 100 years. The South Korean language has changed significantly, particularly the inclusion of many English words which are unfamiliar and unknown to the refugees. A testimony from 'Nothing to Envy' from a waitress describes her embarrassment at being unable to understand as a customer asked for an 'Ice-eu Watu' an imported and South Korean way of saying 'Ice Water'

I used to live in South Korea and found the people there to have an amusing preoccupation with food, for example the local greeting for 'Hi, how are you?' is along the lines of 'Did you eat rice?'. North Korean's are understandably similar in this regard, if not more strongly obsessed with the acquisition of food, considering that they were victims of famine in the 1990s. Many witness testimonies offer shock at the sheer amount of food on offer in South Korea. A particularly heartbreaking observation in 'Nothing to Envy' expresses one refugee's utter shock that a dog was eating a bowl of rice on the street, after a life spent regarding rice as a most precious commodity. The abundance of food is of course welcome, but it can also be traumatic for the North Korean refugees who are discovering the extent of which their former lives have been controlled by lies and are now overwhelmed by choice, consumerism and a completely unknown system."Everything is different here. It is almost impossible to adjust," Cho Myung-chul, a former Communist party ideologue who defected in 1994

http://www.bbc.com/news/magazine-22209894 http://www.theguardian.com/world/2011/jun/30/north-korean-defector-worlds-biggest-prison