“Natural causes” can’t be ruled out. But those include stress, and there’s no doubt Gen. Korobov of the G(R)U was under a lot of that.
https://www.thedailybeast.com/inside-the-mystery-of-the-dead-russian-spy-chief
https://www.radiosvoboda.org/a/29617716.html
https://www.kommersant.ru/doc/3811096
http://www.kasparov.ru/material.php?id=5BF8EA2545010
https://www.radiosvoboda.org/a/donbass--realii/29617362.html
https://www.radiosvoboda.org/a/29617405.html
https://ru.krymr.com/a/sevastopol-v-novyi-god-bez-byudzheta/29613680.html
http://www.rosbalt.ru/blogs/2018/11/23/1748483.html
https://republic.ru/posts/92527