The Chechen wars were particularly brutal on both sides (and the Chechen civil war before the Russians got involved was also brutal). Some people become brutalized in war*, and yes, beheadings happen. I personally saw beheaded bodies in former Yugoslavia. In one case, they had kicked around the head of the old man like a football (soccer ball to Americans). He was an elderly Serb who was caught in the stalled refugee convoy during "Op Storm". We had already rescued his wife and handicapped daughter and I had to tell his wife of his death, but left out the details, and she asked for none. The culprits were Bosnian Muslim V Corps "Hamza" fighters, out of Bihac. This bunch beheaded others, too, and also executed handicapped people in wheelchairs, including children, at a school for the handicapped. Nothing was ever done about any of that, even though UN, OSCE, etc., filed eyewitness reports, including those of officers from NATO countries acting as observers there with the UN and OSCE (OSCE was then called ECMM there).
I know the war in Syria has also been brutal, but am unaware of war crimes committed by Russians there (which does not mean they did not happen, only that I am unaware of them). I have read accounts of horrendous atrocities there, including beheadings and ripping hearts and livers out the bodies of enemy soldiers and eating them, but those only involved local fighters. (Same happened in Cambodia, but as best I recall, they only munched on livers, not hearts, and this was vanishingly rare.) I have seen Russia condemned for the "war crime" of using cluster bombs in Syria. Yet NATO used them during the 1999 bombing of Serbia (including the incident at Nis, which was surely an oopsie, when they killed a number of civilians at a crowded market and also in a hospital and apartment building adjacent to the market):
News and Press Release in English on Serbia about Protection and Human Rights; published on 10 Mar 2009 by NPA
reliefweb.int
Anyway, civil wars, particularly inter-ethnic civil wars, are especially hideous. While the war in Ukraine is not characterized or thought of as an inter-ethnic civil war, it does have an aspect of it. I am hardly brushing off such crimes or making light of them, but war crimes do happen, at least rarely and sporadically, in nearly every war. I don't necessarily believe this beheading of a Ukrainian soldier by a Russian soldier actually happened just because there is a video floating around the net (propaganda abounds), but I can believe such a thing could happen and that a brutalized soldier from either side could be capable of it. I am not defending Russia, mind you, and I hardly think Putin is Mr. Nice Guy (understatement), but I've seen way too many false claims in this war and prior ones, too.
Whether "the West" should have taken stronger action against Russia over the Chechen wars and Syria is a matter of opinion, of course. There have been a number of grisly little wars featuring nasty war crimes over the past 30 years "the West" did not take strong action against.
*Some go nuts, some just get drunk, some wet their pants, and some are able to maintain their clarity and humanity. Nearly all go through a stage of initial shock if it's their first hot war (which might last seconds, minutes, hours or days). When it comes to professional armed forces, the vast majority go by their training and ingrained instinct and simply do their jobs as combatants. A high-ranking officer from a NATO country told me that those capable of compartmentalizing did best in war and were far less likely to suffer PTSD. The way he explained it made sense. I have never been a combatant, so I cannot say I am a proper judge, however. I have been in the very middle of Other People's hot wars and been shot at on more than one continent, but that is hardly the same as the experience of a combatant.
