Putin doesn't want Ukraine, only the Donbass region. However he wants to demilitarise Ukraine and for them to remain neutral so there's a buffer bw Russia and NATO. This is what happens when you poke the bear.
If Putin only wanted the seperatist regions, then he could have pushed troops there (like he did with Crimea) and force an vote on their people as to who they want to be supported by, Ukraine of Russia (who knows on the validity of the voting process).
The reason he has invaded the whole of Ukraine is because Putin believes in the USSR (he has stated this repeatedly) and he believes that those countries that were part of the USSR should be under Russia direct control and influence. He blames the Ukraine on Lenin providing regional control to Ukraine which goes back a long long way in our recent history.
I was watching an interview on BBC News last night with a former CIA operative who was focused on Russia intelligence, and he said that Putin watched the Gaddafi execution on repeat. Ask yourself in the many recent conflicts there have been, who has Putin constantly sided with, those pro democracy groups in various countries, or the despots that aim to suppress and autocrate countries. Who did he support in Libya, Syria, Yemen? He has focused purely on maintaining his autocracy and any signs of dissent or pro democracy uprisings (like the Arab Spring) he wants to quash, as he is scared that similar uprisings could occur in Russia. What happens to his political opponents?
Putin is not the good guy here and I'm astounded that several on here seem to support his actions.