"Insufficient evidence" probably means "If there's any doubt in my mind, then I'll sieze upon that above all else".
In the legal sense, that Russian has been found guilty (publicallg admitted his guilt, even) over killing the man on the bike who had been on the phone. Though if you trust the Ukrainian legal/political system as little as I might with the Russian/Iranian/etc ones, then you could easily dismiss that as pure duress and corruption. (I can quite imagine the path to coury wasn't entirely free of non-conflict physical pressures, which is not ideal, but I wouldn't go so far as to say it was a case of Framing The Guilty Party as a means to an end.
And the sentence is life-imprisonment (and an appeal pending, looking to commute that[1]) rather than the death-penalty that has been described as be summarily (or after interrogation, so not even in the heat of the moment) been enacted upon even civilians 'captured' by Russian units. It's a good high-ground to take (even if his execution might have been cathartic) and bends the relative pressures of "should I fight to the last, or surrender much sooner" upon the respective forces should they end up in a hopeless combat position. As long as the Russian cannon-fodelder know this, of course.
Anyway, back to the proof... There's plenty of credible evidence, as you (i.e. yourself, not the target that needs hyperconvincing) can see. So long as you (i.e. that target person) aren't deliberately critical of the, to your eyes, 'biased' sources. And if you only trust Russian-state media, naturally you can decide to think that every BBC, CNN, AFP, etc source is the actual fabrication. And that's a particularly pernicious rabbithole to be down. You might be able to dig someone out of that. It depends how much is just delusion and how much is active self-delusion, I suppose.
[1] Prediction: Assuming nothing goes wrong (by accident or design) and Ukraine continues to be free, there will be a parole/prisoner-swap in the not too distant future. And if Russia continues to be led by the current propoganda-nose, the guy may even be publically fêted[2] when he returns home for some mythical achievement that's totally not about the trial, but a feel-good thing to assuage the masses, because if they have gone to that much trouble then they should at least use him. But if things go badly, he could be either killed by being in the line of fire as Russia charges past and through his place of imprisonment (highly unlikely they'd try to run a combat sub-mission to extract him, not worth adjusting the scorched-earth tactic of shelling anything they think has Ukrainians in) or just end up lost in the newly-administered system, 'Prisoner X', loved by neither party in the Occupation. Being both Handily Incarcerated Oppressor and Traitorous Yellowbelly.
[2] But possibly privately castrated, hopefully just figuratively, for giving the Enemy the opportunity to put him on public trial.