Well, it's a long story. The assassination could be traced back to the beginning of Nicholas I's reign. Nicholas I was never really meant to be Tsar. His older brother Alexander I reigned during the Napoleonic era and died a few years after Napoleon's final defeat. Alexander had no heir, so the throne would pass to his brothers: either Constantine or Nicholas. Constantine preferred his position in Poland and did not want the throne. However, he did not really make this explicitly clear. Added to this confusion was the fact that it took quite a few weeks to confirm Alexander was indeed dead. So for maybe a month (I'm hazy on the exact timing) there was no Tsar. Nicholas was trying to get Constantine to either take the throne or officially abdicate. Constantine kept delaying and in the meantime some of the nobility began to protest and demand liberal changes. Many of said nobles served in the Napoleonic Wars and consequently saw the liberal reforms of France and the rest of Europe. Upon returning they saw the backwards nature of Russia and demanded change. Nicholas, in the midst of the succession crisis, had no inclination to entertain the protesters demands. The confusion and Nicholas' refusal prompted the Decembrists (the protesters) to basically revolt and march on the palace. Nicholas responded with cannon fire. While this whole little tale doesn't immediately answer your question, it is necessary to give some context.

Nicholas was famously conservative and upon accession immediately clamped down on all liberal reform and agitation. The Decembrists relatively modest demands triggered a harsh and repressive campaign of censorship by Nicholas. Consequently, the educated and those seeking reform turned increasingly towards more radical philosophies and methods. Nicholas eventually left the throne to his son Alexander II. As you noted, Alexander was quite liberal and did indeed want reform. However, his reforms were too little too late. By the time of his rule, the intellectuals and liberals had become far more radical.

Alexander II freed the serfs and limited censorship. He was even considering a constitutional system, but it was all for nought. The liberals of Nicholas' era either became communists, anarchists, or socialists. Consequently, Alexander's reforms meant nothing. The new generation of reformers wanted to completely dismantle the Tsardom. Once Alexander learned of this he tried to clamp down again, but Pandora's Box had been opened. Alexander sent his father's secret police to try to dismantle the radical circles, but that merely sent them towards terrorism. Additionally, Alexander slowed the pace of reform in an attempt to weaken the reformers. Yet, once more, it wasn't enough. Alexander was the victim of multiple assassination attempts and finally succumbed to a bridge bombing. Ironically, the terrorists merely galvanized the conservatives within government who found a new figurehead in Alexander III who went even further than his grandfather.

Some good sources on this are Edvard Radzinsky's Alexander II, W. Bruce Lincoln's Nicholas I and, Franco Venturi's Roots of Revolution. (Sorry for the poor citations, I'm on mobile)