Certainly we should deplore the Russian military intervention in Ukraine and the suffering in the East of that country, but is it worth risking a high likelihood that millions of people will die because the West ends up in a war with Russia? No. How can we save thousands of lives by a war that will kill millions? Yet we are running precisely that risk if we allow Ukraine to join NATO. This will mean that the West will have a duty to go to war with Russia if the Russians refuse an ultimatum to back down - and we should not trust the lives of millions on whether Vladimir Putin can stomach losing face over having to back down.
Certainly Putin is delusional. He is delusional to believe that if he succeeds in capturing large parts of Eastern Ukraine that there will be and end to fighting - it won't, and (unlike Crimea) he will find consistent armed resistance to his forces. Many in the East do not like the Kiev Government, but they do not want to be occupied by the Russian army either. Also Putin is delusional when he goads the west with statements such as: '"Thank God, I think no one is thinking of unleashing a large-scale conflict with Russia. I want to remind you that Russia is one of the leading nuclear powers." See http://www.telegraph.co.uk/news/worldnews/vladimir-putin/11064209/Vladimir-Putin-Dont-mess-with-nuclear-armed-Russia.html
The trouble is that this is precisely what is likely to happen if the Ukraine joins NATO. The road to the first world war was paved with such delusions - on both sides.
The West has long suffered under the logic of 'something must be done' without appreciating whether killing more people will actually make things better. Now I'm not a pacifist, but I do not think that we should take actions that make a bad situation into a catastrophe. Rather we should press the Ukrainians to settle for a solution that will eventually be more or less the same anyway - some sort of Federal relationship involving autonomy for the Donbas Region.
The Putin regime is very unsavoury, that is for sure, and he will come unstuck as he persists with his current strategy. But the best way to deal with him is to let have him have enough rope, so that he becomes more and more bogged down in a quagmire of an unwinnable war inside Ukraine. That way he either will have to come to terms or he will lose, but millions of people across Europe will not die in the process.