There is also the historic and cultural disdain the Russians have for the Ukrainians. They even have a bigoted term for them. In the average russians mind it is unthinkable to lose to Ukraine. It's an old trick. Dehumanize the enemy and your soldiers are capable of anything.

Even if it was a Ukrainian that did the job I would still expect the nukes to fly.

This would need Russian fingerprints or none at all.