Russia not nuking Ukraine had nothing to do with ethics or morals. It's more pragmatic than that. Putin wants to take Ukraine largely intact to control its population and resources, not turn it into an irradiated wasteland. And he knows that use of WMDs would invite economic sanctions from key neutral trading partners such as India.

Also, it's questionable whether Russia's nuclear arsenal would even work reliably. The USA and other nuclear powers spend enormous amounts every year just on nuclear weapons maintenance and testing. Those munitions can't just be stored in a warehouse indefinitely like rifle cartridges and pulled out when needed; they require periodic remanufacturing in order to remain operational at all. Russia is a relatively poor and highly corrupt country so I suspect many of their nukes wouldn't deliver the rated yield if used today. From their perspective an attempted strike resulting in a fizzle would be even worse: all of the diplomatic disadvantages for no military gain.