He's a Russian soldier in Ukraine. It's highly likely that he's a volunteer who signed up for the massive pay given to Russian volunteers (it's illegal under Russian law to send conscripts to Ukraine itself, and this law is followed with rare exceptions).

Ultimately he was trying to murder the Ukrainian soldiers who are in turn keeping me and everyone else in Kyiv safe.

Fuck him. It's a good thing he's dead. Given the opportunity I would have no qualms killing him myself.