Ukraine is not a member of NATO, so options are somewhat limited. Russia intends to use Chernobyl as a deterrent to NATO, according to the report (how, I do not know). It is also not straightforward to put a NATO ally right on Russia's vulnerable flank (it's endless plains and flat ground from Ukraine to Moscow, which has always been a source of strategic vulnerability for Moscow), so admitting Ukraine to NATO was never a concrete possibility. In fact, the US has had to tread very carefully simply to place missiles that are capable of carrying nuclear warheads in Poland, which is a neighbor of Ukraine.
Russia had a buffer state that was content to do its bidding in Yanukovich et. al. but after the Ukrainians overthrew their corrupt government and made moves to establish a genuine western-style democracy with rule of law, Russia were forced to shore up their vulnerable flank. This is part of that process. But they now face bad consequences, including neutral Finland and Sweden deciding to join NATO, perhaps Georgia also, and Germany (and Europe in general) starting to look for alternate energy sources in earnest (Russia supplies a huge percentage of Europe's energy needs, and is also a huge supplier of many rare earths and raw materials that are critical to the Semiconductor Industry).
EDIT: Belarus is also a client state of Russia's, and there has been unrest there, so perhaps Russia is sensing that they might slip as well. Putin is desperate to do whatever it takes to stay in power in Russia, and these things play into that as well.
Russia had a buffer state that was content to do its bidding in Yanukovich et. al. but after the Ukrainians overthrew their corrupt government and made moves to establish a genuine western-style democracy with rule of law, Russia were forced to shore up their vulnerable flank. This is part of that process. But they now face bad consequences, including neutral Finland and Sweden deciding to join NATO, perhaps Georgia also, and Germany (and Europe in general) starting to look for alternate energy sources in earnest (Russia supplies a huge percentage of Europe's energy needs, and is also a huge supplier of many rare earths and raw materials that are critical to the Semiconductor Industry).
EDIT: Belarus is also a client state of Russia's, and there has been unrest there, so perhaps Russia is sensing that they might slip as well. Putin is desperate to do whatever it takes to stay in power in Russia, and these things play into that as well.