Yeah sure . Neo Nazis are saints
No one are saint in war. Everyone have been in war know that, If you don't do stuff that you aren't proud of, you can't survive
That said, that particular incident is really Russian disinformation.
If you watch the entire clip, you would see that the Russian wounded was still moving, and the lone Ukrainian soldier went to check on him, and at 0:12 that Ukrainian soldier was being shot at, now I don't know if he has been shot at by the Russian under that APC, but you can clearly saw the dust kicking up from the bullet hitting around him.
The second "Close Up" scene we can see the Russian soldier no longer moving, suggest that this happened after the first bit. And he throws a grenade (With Grenade in hand already, which mean that is what he intended to do) to underneath the APC to clear that position
Most likely the gunfire came from under the APC, and if so, then either the Wounded Russian soldier fight back or someone hidden with that wounded and we can't see who do the fighting (Most likely that way) , either way, that render the encounter legitimate.
One explanation of this video is that Ukrainian soldier go and check on the APC and found out there are some Russian hiding under it, he drop a grenade in that position, in turn the Russian shot at him, and the grenade exploded, incapping the people underneath, then get close again to clear that position.
Otherwise no other explanation can explain the shot at around 0:12 except it either come randomly or from the Ukrainian side, which is not likely.