I'd be willing to bet that their anti-corruption police are just as corrupt as their normal police.

I'm guessing that these corruption police are just for appearances, or to be used to get rid of anything that Putin himself labels as "corruption", since they work for him.

But since everyone is corrupt in Russia, anti-corruption means you can go after whoever you want to get out of the way.

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.

The knife missile could take care of both problems at once.

Here's how I picture it. Half way through in the middle of the row with a 1 to 2 root crater marking the transition between the neat grass and the unmowed grass

Because they drop grenades on their territory and use kamikaze drones on Russia. And again, financial police? They could spend grenade much better.

I'm guessing Ukraine or Putin doing it to make it look like Ukraine.