Or, and this might be a crazy theory, if he was only 1k feet from the Ukrainian border, maybe, just maybe, Ukraine got him, since he would be a high profile target and Russia is currently at war with Ukraine. He worked for Putin, so despite the “anti-corruption” title, the chances of him actually going after real corruption is slim to none. “Corruption” in this case probably just means “whoever Putin is mad at atm”. Even if he was killed by his own team for whatever reason, the one behind it would most likely be Putin himself.