Or he could shoot some down "accidentally" to see what price NATO would be willing to pay and suddenly this becomes a NATO Russia conflict with Russia seriously outgunned which in Putins mind could justify a battlefield nuclear response.
These things could happen without a NFZ as well. Putin could get desperate inside Ukraine and go for the Hiroshima style kill shot in the hope Ukrainians would instantly surrender. We have to broaden the possibilities beyond the usual Article 5 fearmongering and consider the possibility that we are already in WW3, which case its important to broaden the ROE.