“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
http://www.dsnews.ua/world/pomenyat-doktrinu-razreshit-li-sebe-putin-preventivnyy-23112018080000
https://www.bbc.com/russian/features-46291929
https://www.gazeta.ru/politics/2018/11/23_a_12069265.shtml
https://day.kyiv.ua/ru/blog/politika/smert-glavy-razvedki-pochti-po-gogolyu
https://theins.ru/politika/128525
https://lenta.ru/articles/2018/11/22/ch_v_k/
https://www.depo.ua/ukr/svit/kinec-korobova-chomu-tak-chasto-ginut-putinski-shpiguni-20181122873929
http://www.dsnews.ua/world/golosovanie-v-interpole-i-ostrovnye-gosudarstva-21112018220000