There was no "soviet empire." The communists spent the decade prior trying to form an anti-Nazi coalition force, such as the Anglo-French-Soviet Alliance which was pitched by the communists and rejected by the British and French. The communists hated the Nazis from the beginning, as the Nazi party rose to prominence by killing communists and labor organizers, cemented bourgeois rule, and was violently racist and imperialist, while the communists opposed all of that.

When the many talks of alliances with the west all fell short, the Soviets reluctantly agreed to sign a non-agression pact, in order to delay the coming war that everyone knew was happening soon. Throughout the last decade, Britain, France, and other western countries had formed pacts with Nazi Germany, such as the Four-Power Pact, the German-French-Non-Agression Pact, and more. Molotov-Ribbentrop was unique among the non-agression pacts with Nazi Germany in that it was right on the eve of war, and was the first between the USSR and Nazi Germany. It was a last resort, when the west was content from the beginning with working alongside Hitler.

Harry Truman, in 1941 in front of the Senate, stated:

If we see that Germany is winning we ought to help Russia, and if Russia is winning we ought to help Germany, and that way let them kill as many as possible, although I don’t want to see Hitler victorious under any circumstances.

Not only that, but it was the Soviet Union that was responsible for 4/5ths of total Nazi deaths, and winning the war against the Nazis. The Soviet Union did not agree to invade Poland with the Nazis, it was about spheres of influence and red lines the Nazis should not cross in Poland. When the USSR went into Poland, it stayed mostly to areas Poland had invaded and annexed a few decades prior. Should the Soviets have let Poland get entirely taken over by the Nazis, standing idle? The West made it clear that they were never going to help anyone against the Nazis until it was their turn to be targeted.

It was not only military but also economy cooperation. Soviet union was selling oil and resources, Germany was selling machinery and equipment.

Correct, the soviets desperately needed finished goods that the west would not sell them.

Also before the pack, due to isolation of Germany after WW1, Germany cannot have a lot of tanks, and army and so on, also western countries were not very happy about cooperation with Soviet Union.

Treaty of Rapallo 1922. Not only normalizing relations between Soviets and Germany, but also allows Germany to secretly train army in the Soviet union. After Hitler came to power in 1933, relations cooled sharpy, and military cooperation was reduced.

Yep, the west destroyed Germany and invaded the USSR, so it was unsurprising that pre-Nazi Germany had ties to the USSR. In fact, the west loved the Nazis more than pre-Nazi Germany.

Of course, after Germany attacked Soviet Union, all the cooperation stopped, and both sides were killing each other POV without any mercy

Yep, the soviets beat the Nazis. Communism is inherently anti-fascist, and fascism anti-communist.

Remember, one of the key things that build german power, was cooperation with Soviet Union. Before Hitler it was very big economic and military cooperation, then military element was reduced (but not stopped, one of the result was Molotov-Ribbentrop pact) but the economic cooperation between Nazis and Soviets was still going on for a long time.

After Hitler took power, it was the west that cooperated and traded with the Nazis the most, even during WWII.