Since this report comes from the South Korea’s National Intelligence Service I give it a little more credence than reports that come from defectors:

Hyon Yong-chol (circled), minister of North Korea’s People’s Armed Forces, appears to have his eyes closed during an event for military officials presided over by Kim Jong-un (center), held on April 24 and 25, in a photo published by the North Korean daily Rodong Sinmun on April 26. South Korea’s spy agency reported Wednesday that Hyon was executed after possibly being accused of having dozed off late last month. [Rodong Sinmun]
North Korea executed its defense chief late last month, South Korea’s spy agency reported Wednesday, the latest chapter in leader Kim Jong-un’s reign of terror.

A firing squad executed the defense minister with an antiaircraft gun, the agency said.

Four-star Gen. Hyon Yong-chol, minister of North Korea’s People’s Armed Forces, was executed around April 30 at a military academy in Pyongyang with hundreds of senior military officials watching, said the National Intelligence Service (NIS), citing multiple sources but refusing to disclose them in detail.

The spy agency said Hyon could have been charged with treason but it was likely that he was executed because he showed some sign of disrespect to leader Kim Jong-un rather than plotting a rebellion.

The NIS said Hyon had expressed dissatisfaction with Kim Jong-un’s governing style, without elaborating further. The agency said Hyon dozing off at a military event held on April 24 and 25 in Pyongyang might have sealed his fate.

In a photo published by the state-run daily Rodong Sinmun on April 26, Hyon appeared to have his eyes closed during an event for military officials presided over by Kim Jong-un, who was seated in the center in the front row. Hyon was seated to Kim’s left.  [Joong Ang Ilbo]

You can read more at the link, but it is a good thing that the US military doesn’t have this execution policy when people nod off in hours long meetings because we would not have a military.