According to an unidentified and exclusive source of the Chosun Ilbo, the former top nuclear envoy with the U.S. along with four other foreign ministry officials in March were executed after a failed summit between Kim Jong Un and Donald Trump in Vietnam.
Kim Hyok Chol (his biogram is available here), who led working-level negotiations for the February summit in Hanoi, was supposedly executed by « firing squad » after being charged with espionage for allegedly being co-opted by the U.S.
I would carefull analyze this information as previous reports in South Korea media about senior North Korean officials being executed have proven false. For instance, Kim Jong Gak, a former member of the WPK Politburo and the National Defence Commission, was said to be purged in 2013, but reappeared later as being the dean of the Kim Il Sung University.
The source of the Chosun Ilbo also indicated that other officials were also purged with Kim Hyok Chol. Kim Yong Chol was sent to a labor camp in the Jagang Province. Kim Sung Hye, Kim Hyeok Chol’s assistant and Shin Hye Young were sent to two different political prisons.