Russian media is the best 🤦 Reporting about this sentence, they classified it as “politically motivated” and compassionately quote words of Petrovsky in the court:

‘I had no part in it. I did not fire a single shot,’ Torden said in response to accusations of participation in the shootings. According to the prosecution version, the group shot 22 Ukrainian soldiers and wounded four others on 5 September 2014. As for the phrase ‘There will be no mercy, and we will not take prisoners’ uttered by him on the video, it was Thorden who repeated the words of the commander. Moreover, the words ‘there will be no mercy’ were not in the video. He also emphasised that wounded prisoners of war or enemies should be treated with respect.

Note that this short paragraph hints what the evidence is - in reality it’s videos filmed by Petrovsky himself with dead soldiers and saying things unambiguously indicating he participated in the killing. Petrovsky was one of the most blood thirsty and radical members of “Rusich” and he was proud of it, so he published tons of videos and photos showing “no mercy”. This is why this sentencing was so easy…