in fact the entire point of mechanized armored maneuver is to enclose large swaths of enemy fighters
So do tell, genius - what happens to those “large swathes of enemy fighters” after you’ve enclosed them, eh?
These are baby questions about war, go do some basic reading about it.
What happens is hopefully for everyone's sake they surrender.
Maneuver seeks to inflict losses indirectly by envelopment, encirclement and disruption, while minimizing the need to engage in frontal combat. In contrast to attrition warfare where strength tends to be applied against strength, maneuver warfare attempts to apply strength against weakness in order to accomplish the mission.[1]
https://en.wikipedia.org/wiki/Maneuver_warfare
A common thread runs through these critiques: the relationship between tactics and victory. The maneuver versus attrition dichotomy treats some tactics as inherently bad and others as inherently good. What matters is whether tactics can translate to strategic effect that serves to achieve the political end state, a relationship rendered opaque by the imposition of the operational level of war. Finally, the actions after tactical victory is achieved determine strategic effects. Tactical victory is insufficient as an end state.
https://www.mca-marines.org/gazette/maneuver-warfare-a-defense/
If Ukraine were to start slaughtering surrounded, defeated, starving russian fighters not only would it be a war crime, it would also NOT break the fighting will of russians and russian fighters, it would do the opposite and strengthen their resolve to fight to the bitter pointless end, that is why Ukraine isn't doing it... on top of it being inhumane and also resulting in traumatizing their own soldiers.
None of this is to say Ukraine is perfect, war is a disgusting thing and everything gets messy, but as a pattern no, Ukraine is not needlessly killing russians when they attempt to surrender that would backfire spectacularly for Ukraine on so many levels it makes my head hurt. I have also never seen credible evidence Ukraine is systematically mistreating their prisoners of war, conducting genocide and human rights crimes against occupied territories nor systematically targetting russian civilians in acts of mass terror that do nothing to change the frontline reality russia is losing... all things it has been well established russia is doing by independent international entities.