According to the Associated Press, in November President Yanukovych withdrew Ukraine from a deal with the European Union, and accepted a $15 billion bailout from Russia. Both Western Europe and Russia want to have a strong sphere of influence in this former Soviet Republic. Protestors also accuse Yanukovych of "foot-dragging" on Constitutional reforms that would limit his power.
Most of the protestors want Ukraine to pursue closer relations to the West, especially in light of Russian policies starting to feel dangerously similar to old Soviet Union policies. But President Yanukovych continues to pursue close relations to Russia. Since President Yanukovych isn't listening to their voices, they are "raising their voices" by rioting. Monday, Feb. 17, Russia resumed bailouts of Yanukovych's administration, and it seems Yanukovych has decided to put his foot down, and the protestors aren't going to leave Independence Square without a fight, hence the escalating violence.
As for whether Yanukovych bears any responsibility for the bloodshed, its less clear cut. A common cliche about democracy is that "no one gets exactly what they want." It is quite possible that Yanukovych has good reason to pursue closer relations with Russia, in which case the protestors are solely to blame for the situation. It is also possible that Yanukovych has no justification for closer relations with Russia, in which case he is to blame for the violence. At the very least he owes the Ukrainian people, and the world, an explanation for his policies so that they, and we can judge the reasonableness of both sides.
According to CNN, the protests were caused by the Ukraine president's rejection of an EU trade pact called the "Eastern Agreement" and subsequent decision to enter into a pact with Russia. Ukraine is fraught with various divisions that essentially boil down to the fact that some people want closer ties with Russia, and some want closer ties with Europe.
Broadly speaking, those that want closer ties with Europe tend to be younger, more urban, and more liberal while those who want stronger ties with Russia tend to be older, more rural, and more conservative.
Another major factor behind the protests is the widespread belief by the protestors that the parliamentary elections following the election of President Viktor Yanukovych were corrupt and did not represent the will of the people. Outside observers agreed, and following the elections, the High Representative of the Union for Foreign Affairs and Security Policy, Catherine Ashton and European Commissioner for Enlargement and European Neighbourhood Policy Stefan Fule stated "We express our concern about the conduct of the post electoral process, which was marred by irregularities, delays in the vote count and lack of transparency in the electoral commissions"
The United States, and other countries agreed that the elections were compromised, and the opposition in the Ukraine has been upset about the elections since they took place. This feeling that the will of the people has been denied is possibly the strongest undercurrent in the current revolution that is taking place.