That is an unanswered question. Someone killed them.
Was it someone sending Kim a message or was Kim calling the shots. NK is a military state, the military has a ton of power.
Right before this summit Kim managed to purge a few military leaders. The kind that would be puppet masters over him. There are a lot of questions that need to be answered. The big one being was Kim a puppet of the military that has received outside help to gain some power or is he the real power? There are indications either could be the truth.