It was "the devil you kinda have to trust" deal. Due to geography and resources, the Allies needed that eastern front covered, and what do you know, the Soviet Union fit the bill.
Stalin was just the leader
not invading Western Europe at the time, had there been any other large power in the area, I'm sure the Allies would have done secret deals with them. Stalin was a duplicitous and treacherous man with power, but he did have power. The Allied leaders didn't really trust him at any point and he certainly didn't trust them (paranoia is a dictator's best friend). At Yalta, it was more agreements not to get into each others way than anything else, it wasn't a coordinated and friendly meeting. Earlier on during the war, it was Stalin who really took the initiative, and the Allies just realized he was going to play a part on their side by default, since Hitler had already broken his agreement with Stalin:
Wikipedia said:
While the Germans pressed forward, Stalin was confident of an eventual Allied victory over Germany. In September 1941, Stalin told British diplomats that he wanted two agreements: (1) a mutual assistance/aid pact and (2) a recognition that, after the war, the Soviet Union would gain the territories in countries that it had taken pursuant to its division of Eastern Europe with Hitler in the Molotov–Ribbentrop Pact.
The British agreed to assistance but refused to agree upon the territorial gains, which Stalin accepted months later as the military situation deteriorated somewhat in mid-1942. By December 1941, Hitler's troops had advanced to within 20 miles of the Kremlin in Moscow. On 5 December, the Soviets launched a counteroffensive, pushing German troops back 40–50 miles from Moscow, the Wehrmacht's first significant defeat of the war.
http://en.wikipedia.org/wiki/Joseph_Stalin#Pact_with_Hitler
Hitler takes the cake for maliciousness and genocide, though (note: a "maliciousness cake" would not be delicious, nor would the name be good for marketing...), not that there's a competition or award ceremony for pure evil. He would get second place for the time period, though.
The relationship between the other Allies and Stalin/the Soviet Union was just out of perceived necessity. War and politics makes for strange bedfellows. Had Hitler not pressed his luck - though he might have been clinically insane at some point there, and not just evil - by turning against Stalin, things might have played out very, very differently.