How far was Stalin personally responsible for the development of the cold war in Europe in 1949?

Stalin was not the only factor in the rift between the Soviet Union and the US, and in fact the rift had existed ever since the founding of the USSR.  But Stalin did have a great deal to do with how the Cold War came about, and part of that was due to his political goals and paranoia.

Lenin and those around him considered all "imperialist" and capitalist nations their enemies from the beginning.  Long-term conflict between the World Revolution as envisaged by Lenin and the Western powers was inevitable.  The entire purpose of the Revolution was to destroy the capitalist governments, after all.  That is why the US found itself landing troops in Siberia in 1918, and why Lenin had expected them to.  Stalin's Soviet state was in total idealogical conflict with all other political systems.

Another part of the reason for the Cold War was simply the weight of Russian history. Russia had been an expanding empire since the 10th century, and always felt itself surrounded by enemies.  The experiences of the Soviet government through the Revolution and Civil War had not revealed friends in the world at large, and the Second World War was a horrifying ordeal.  Stalin joined forces with Britain and the US only because he needed to, to survive the Nazi onslaught.

Stalin and the intelligence services of his government spent a great deal of effort preparing for the period after German defeat.  He had no intention of giving up territory the Red Army took, but either absorbed the lands or set up puppet governments.  Spy rings were in place by the end of the war, including the "Cambridge Five" group in England with Kim Philby.  Stalin knew all along that after the war inevitable ideological tensions would cause either actual war with the West or a state of armed truce complicated by small, proxy wars and espionage operations.

His delay entering the war against Japan was a ploy to let America be weakened by a protracted campaign in Japan and possibly China.  With America weakened by that struggle, Britain economically ruined and Germany destroyed, the USSR would have been the big winner of the Hitlerian Wars.  That didn't work out, but he did foment a war between the US and China in North Korea.  By doing so he not only weakened a rival Communist "brother-state" but reinforced in the minds of American right-wingers the view that Communism was a monolithic threat, thus increasing the drift into McCarthyism.

Stalin himself was not only a ruthless and successful dictator, but a man suffering increasingly from paranoia.  His ideas on the expansion of Soviet territory and power, and his views on the World Revolution made the Cold War an inevitability.  His deep-rooted suspicion of everything and everyone only made his distrust of capitalist societies more severe.  The Cold War would have happened anyway, but Stalin's method of rule and personality went a long way toward making it as militarist and espionage-driven as it was.  A different Soviet ruler might have been more diplomatic about things, but not Stalin.