I wouldn't say that it can't happen...but if it does happen, then I'm actually going to finally agree with every 'expert' that called Putin as certifiably insane on the day the war started. him nuking Kiev would definitely tell the West: "if I was willing to nuke a city this important to me, what do you think I'll do to yours?" but it would also tell everyone in Russia something that would not benefit him at all - "I respect nothing of all the bullshit I yapped about the glory of slavs before now and could do this to any of our own cities too." and no matter who you are in a position of power, you do not last long with that kind of statement.