I am relatively certain that the accusations Ukraine is making are true. Ukraine would risk a great deal of their international support if they were found to be staging atrocities—they are only likely to do something like that if they were being defeated and having trouble garnering support. Your willingness to do something like that is a function of how much you have to gain vs. how much you have to lose. However, I suspect many people will be suspicious of Ukraine’s claims because they can’t imagine what would possibly motivate Russia to behave that way. For those of you who might be struggling to understand why Russia did this, here are some possible reasons:
1) Revenge against "Nazis" who fought in the Donbas.
There are reports that those who were executed were those who had tattoos suggesting they had participated in the fighting in Donbas. Given the intensity of Russian propaganda regarding what has occurred there, it is likely many of these Russians just thought they were "punching a Nazi" or, rather, shooting a Nazi in the back of the head.
2) They were targeting men with military experience in order to reduce the threat of guerilla action.
This would also explain why they tended to target people with military tattoos. It might also explain some of the random killing of civilians who were out and about---scare civilians into hiding and you can more easily target guerillas and saboteurs who will not be able to hide amongst them.
3) The Russian government wanted its commanding officers to "get their hands dirty" in order to stave off regime change.
In a manner not all that dissimilar from street gang inductions, Putin may have wanted to force his generals to "make their bones" so that they would likely face war crime tribunals if he were overthrown. What is genius, albeit evil genius, about this is it would make the Ukrainian government and the international community indirectly serve as Putin's enforcers: By threatening war tribunals, we ensure Putin's men are never incentivized to turn on him. The Hague would, in effect, serve to prop Putin up by acting as a kind of assassination or punishment squad against his military if Putin were removed from power.
4) Kadyrov's men lack discipline and were known to have occupied the area in just before the pullout. They may have also been instructed to carry out killings in order to indirectly terrify people in occupied Donesk and Luhansk.
That the Chechen army is being used to terrify the population into submission, and to punish populations that rebel by offering them "something worse", is no surprise: It is the chief reason they have been tasked with hunting down Zelensky and the other heads of government. Kadyrov is known to be Putin's hit man---someone he gives the task of carrying out especially sensitive assassinations to, where Russia needs some element of deniability.
5) Individual Soldiers’ Financial Gain and Sexual Pleasure
Of course, simple rape and robbery may have motivated a significant number of these killings. The Russian military has a long history, going back at least to the Second World War, of engaging in this sort of behavior. The Red Army is known to have raped women en masse during the final days of the Second World War and during their occupation of Berlin.
6) To create a "brutal battle" narrative
To up the casualty figures with a "trail of tears" to make it look---not like the lopsided victory it was---but a vicious fight with losses on both sides. In short, Russia may be committing atrocities in a deeply misguided attempt to save face. Indeed, they may try to count military age males as enemy kills because technically Ukraine has claimed that all such men should be rendering military service.
7) To remove dissidents who might speak out against the Russian occupation or document Russia’s behavior during it.
We all know that the Russian government does not tolerate dissidents even in their own country; why would they tolerate them in enemy occupied territory. Anything from organizing a protest to writing a blog might easily provoke their ire and lead to an execution.
So, there are reasons to suspect that these killings are real: Russian soldiers, and the Russian government, had real motives for committing them, and Ukraine has everything to lose—including the war itself—if they were found to be lying.