If Putin starts using tactical nukes and/or chemical weapons, then IMO the United States/NATO is going to have to lay down the law in some way and make him stop, because otherwise it shows too much fear to Putin and to our enemies that we will even let him get away with mass scale slaughter so long as it isn't a NATO country. Which then means he could attack a NATO country or try to annex part of one and just say, "Stop me and I nuke you." You can't live in constant fear like that. The Chinese, Iranians, and North Koreans are most definitely watching and will take note, so the West must stand strong and firm and be ready to fight if it comes to it. I do not mean that out of any gung-ho war attitude, I mean that out of the concern that otherwise, not only would we be allowing mass slaughter to occur, but we'd also be giving carte blanche for China, Iran, and NK to start behaving with the same tactics. It is one thing to attack a nation, it is another thing to start using nukes and chemical weapons.
I do not think the West should say anything right now about this; if Putin hasn't said anything about such weapons, then we should keep quiet so as not to provoke him. But if Putin himself starts saying he may use them, I think the West should immediately law down the law and say basically that if he escalates to that level, he will be stopped. You cannot have someone lobbing nukes and chemical weapons with no stopping. Showing such fear and leniency, IMO, is what can start a domino of events that leads to a full-on war that is much larger than anyone imagined.