Footage making its rounds on social media shows an incident in which a squad of Russian soldiers is surrendering to Ukrainian troops. They come out one by one with their hands up and lay face down. However, at some point a Russian soldier steps out and opens fire on the Ukrainian troops. The video then transitions to a severely wounded Ukrainian soldier and then aerial aftermath footage of the area showing the entire squad of Russians dead.

The incident has both Ukrainian and Russian supporters accusing each other of war crimes, and there was certainly a war crime committed here... by the Russian soldier that opened fire. His illegal act, defined by the rules of war, is called "perfidy" which are acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the law (of war), with intent to betray that confidence … Perfidious acts are prohibited on the basis that perfidy may damage mutual respect for the law of war, may lead to unnecessary escalation of the conflict, may result in the injury or death of enemy forces legitimately attempting to surrender or discharging their humanitarian duties, or may impede the restoration of peace …

While I'm not a military tribunals judge, the Ukrainians killing the already surrendered Russians in this situation would be exonerated by the wording of the laws that explain why perfidy is illegal in the first place; "Perfidious acts ... may result in the injury or death of enemy forces legitimately attempting to surrender..." Essentially, perfidy is banned by the rules of war because this is what will happen.