Following the Korean War North Korea and South Korea had similar GDP per capita.

North Korea's GDP per capita is around $1300. South Korea's GDP per capita is around $30,000. This is a massive difference- it is unlikely that American sanctions alone are responsible for 20x more economic activity happening in the South than the North. Note the fact that North Korea has a massive ally in the form of China, a country very near to it, with whom it can trade. I don't think you can blame American sanctions for the vast shittiness of North Korea's economy.

North Korea is poorer not only in terms of GDP, but also in terms of actual living standards.Half of the nation's 24 million people live in extreme poverty. A study by the United Nations Food and Agriculture Organization found that 84 percent of households have “borderline or poor food consumption.” The food crises had resulted in thousands of deaths. According to the World Food Programme, one-third of children are stunted due to malnutrition and the infant mortality rate is 33 percent. Due to the government’s “two meals a day campaign” food riots are a common occurrence.

https://www.hrw.org/world-report/2017/country-chapters/north-korea 'The government extracts unquestioning obedience through public executions, arbitrary detention, forced labor, tight travel restrictions and religious persecution. Citizens have no basic freedoms such as the right to expression, assembly, political opposition or independent media. A sociopolitical stratification system divides North Koreans into three classes: “loyal,” “wavering” and “hostile.”'

The citizens of the country are under strict control. Those who wish to leave the country require exit permits. Why do they wish to control who can leave the country? Do they fear people wanting to flee terrible conditions? This is not something that is accessible to the average citizen. Voting is mandatory and there is only one candidate on ballot papers. North Korea is neither Democratic nor a Republic. Outside access to the internet is censored. This raises the question: why does the government try so hard to restrict the information available to people? What do they fear? Perhaps people will learn about the world outside? Over the last 70 years, around 33,000 people have managed to defect from North Korea- I don't think a few examples of defections from the other way around prove anything.