Are you arguing there will be a PR issue with Ukraine killing Russian soldiers on the battlefield, or are you suggesting Russians (and their socialist/communist/leftist/right wing/etc supporters in the US) will be making these arguments? That's a pretty laughable suggestion IMO.
A bit of both. I vividly remember the shift in perception for the Iraq war after it wasn't just planes and missiles bombing tanks and palaces but actual street fighting with people (mostly looters) lying dead in the streets.
The images of scores of dead Russians will shift the perception of this war, especially in Russia itself but also in EU countries like Germany and Italy that prefer a diplomatic settlement with Russia over all out war.
Nor will Ukraine run out of weapons as long as the US keeps sending shit. I see no indication that lethal aide will end anytime soon. Flip side does Russia have the vehicles and ammo they need? Begging North Korea for weapons doesn't present confidence...
Ukraine will use much more ammo against a larger Russian force. The US supplies Ukraine with specialized lethal aide but it's already having trouble to supply enough AK 47 ammo and other Soviet-style ordinance that he Ukrainian army and especially the territorial defense forces are mostly familiar with. Some European countries have already signaled that they're out of stockpiles to send. More is being produced and Ukrainian forces are being trained to use new weapon systems but it will take time to reach the front lines.
Russia may find it difficult to field enough vehicles and tanks after they suffered heavy losses but they aren't short on rifles and grenades to hand-out to conscripts. Getting the stuff to the front lines is really the big difficulty for both sides as they've become experts in targetting each others ammo dumps.
Guessing Putin doesn't give a shit that once the war is done and dusted there will probably be decades of terrorism on Russian soil 
Putin's realizes that his own survival hinges on "winning" this war. Which is why he will not end it, nor give up.
His supporters know that without Putin they're all dog food for revolutionaries or ultra-nationalists.
