“my guess is these ‘wild brigade’ Chechens are pro-Russian and looking for some payback.”
You are most likely correct...
The now-dead Sashko Bily/Oleksandr Muzychko, a Right-Sector Leader at the Maidan protests, and definitely involved in the violence, was wanted internationally for the torture murders of some 20+ captured Russian POWS in a Chechnyan jail.
This was a BIG issue to the Russians after the coup, and was likely the reason why Avakov had him killed.
OOPS... I mean, per Avakov, he shot himself... TWICE... while fleeing police outside of a nightclub.
Russia accused him, without any evidence - and many years after the supposed incidents alleged - of crimes in Chechyna.
Yet, Russia never sought extradition.
However, you yourself are excitedly bragging that Muzychko was murdered by Russians extrajudicially on Ukraine soil.
That tells us a lot about who you are and what you value.
Who ever killed him...did the world a favor.