Putin knows that no one is going to prevent him from taking Crimea (which was not traditionally a part of Ukraine anyway) in any meaningful way. Europe depends on Russia to stay warm in the winter, and a lot of cars run on LNG from Russia as well. Europeans are not going to boycott Russia gas and oil. Due to logistics (gas pipelines cannot be run under the ocean) gas from the US is unlikely to replace gas from Russia.
It is not a matter of justice. Many Ukrainians in the Eastern part of the country see that Putin is a strong leader and Russia is increasingly prosperous, while Ukraine so far has only elected corrupt and incompetent leaders (ostensibly of two political persuasions) and the economy is in a shambles. Crimea is backward, most of the people are not Ukrainians, and the economy depends on tourism. There is not a lot of Ukrainian tourism because Ukraine is in a state of economic collapse: Russian tourists are the ones that many of Crimea's inhabitants would prefer.
Many Ukrainians would prefer to switch from being impoverished Ukrainians to being richer Russians. Ukraine did pretty well under the Soviet system. It has been mostly a corrupt disaster as an independent nation. And there is not all that much difference between Ukrainians and Russians anyway. They have different accents, but the understand each other. They have different recipes for kreplaks, kvass and borscht. The culture is not greatly different: think Manitoba and northern Minnesota.
Putin knows what he can do and what he cannot do. He understands Russians and Ukrainians quite well. I am sure that if I were Russians I would oppose him, but he is no dummy, and he knows what his limits are. And I am not a Russians and do not think like a Russian.