“Probably so, though India secret services recently tried to assassinate Sikh separatists in both Canada and the USA...”
Which would be extrajudicial killings (murders) unless Indian law allows it (in which case the killings would be legal according to Indian law, but probably not Canadian or US law).
“...and the Mossad wouldn’t hesitate to kill Hamas leaders who sneaked across our porous border.”
Clearly legal per Israeli law; but they could run afoul of US law.
Which brings us back to the initial post: If the assassination was legal under Ukrainian law, then there should be no adverse legal repercussions in or from Ukraine against the perpetrators; if it is not legal under Russian law, then Russia could bring charges.
Which would bring up an interesting dichotomy: Russia clearly thinks it’s okay to murder its dissidents the world over, but denies other countries the same right. Popcorn time.
The spirit of Machiavelli lives on all over the globe.