“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://echo.msk.ru/blog/mbk313373/2321320-echo/
https://www.kommersant.ru/doc/3811107
https://www.gazeta.ru/politics/2018/11/22_a_12069037.shtml
https://www.gazeta.ru/politics/2018/11/23_a_12069925.shtml
https://republic.ru/posts/92527
https://newtimes.ru/articles/detail/173196
https://day.kyiv.ua/ru/blog/politika/smert-glavy-razvedki-pochti-po-gogolyu
http://www.rosbalt.ru/russia/2018/11/23/1748499.html