He not only had a supermarket chain in the Donetsk and/or Logansk areas but also a long-ongoing feud with one Arthur Emelyanov who was a judge and has fled the country and is under investigation for fraud and kidnapping.
Due to this feud, it was suspected previously, he also got theats by mail before (pic, a note & a laser pointer).
From what I read in russian/ukrainian news, it seems unlikely that it has anything to do with his one-year role at Naftogaz.
https://www.rbc.ua/rus/styler/ubiystvo-trehletnego-syna-deputata-kieve-1575270647.html
https://www.facenews.ua/articles/2019/331603/