Good idea. I've heard that nuking cities is a very effective way of forcing a stubborn enemy nation to surrender.

It's also preferable to a prolonged invasion and conventional bombing campaign, simply in terms of number of casualties. The number of people killed by the two nuclear bombings it would take to force Ukraine to surrender would be significantly smaller than the number of casualties incurred by a drawn out conventional invasion.

Seems like the only moral course of action for Putin to take.