I think it's worth remembering that when there's war, there's war propaganda.
If they are going on and on about some terrible thing one side supposedly did - if that thing would make zero sense for them to have done, not advanced their goals in any way, but makes them look as if they are doing evil things just to be evil...
... then the story is fake.
No, just because you can't comprehend motives doesn't mean it didn't happen. You are assuming people are acting rationally in war, but they absolutely don't.
There was no rational reason to piss on prisoners in Abu Graihb, but they did. There was no rational reason to do the holocaust, but they did. There was no rational reason to club to death anyone wearing glasses, but they did.
People are good at othering others, and stop thinking of them as human beings worthy of respect. Politicians take advantage of this, and have wildly succeeded in both Russia and Ukraine at dehumanizing the other.
Thus when a Russian soldier is frustrated because he is being shot at by javelins and can't find them, why not take it out on those bicyclists? It's not like they're really human, they're just a bunch of Nazi Animals, who do need to be killed to denazify the area anyway.
Or a Ukrainian seeing a captured Russian while frustrated at the news of his countrymen dying: Oh, that prisoner? Probably also committed massacres. Might as well give him a shot in the knee so he can't do it anymore. Or why not just kill him?
So no, illogical and stupid atrocities happen all the time.
EDIT: I should actually amend my statement: the reasons were rational. Evil, but rational. Basically, they thought it would be fun/let out stress/make them feel better, and either placed a high value on gratification now or just don't think they would be punished.