Why do you think so many people on the Marxist left defend Stalinism?

Is it simply over-correcting in response to western anti-communist propaganda? I'd like to think it's simply memeing for memes sake, but it feels t…

Could it have anything to do with saving the entire world from the Third Reich by being the leader of the only military that actually stood up and ended the Nazi reign of terror, defeated 80% of the Nazi military, marched all the way to Berlin, and through Berlin, before any allies showed up, and liberated the concentration camps?

No. It’s probably vibes.

Standing up only after you are betrayed isn’t quite as impressive as standing up because it’s the right thing to do. Soviets would have been happy to sit and do nothing until the Nazis started threatening the Soviets. But, still, yes the Soviets kicked ass
Hitler threatened the USSR over a decade before he invaded. He wrote it in Mein Kampf. It was out in the open. Stalin attempted to get Western Europe to take the threat seriously, but Western European leaders understood that the primary target of the Third Reich was the USSR and they all wanted the Third Reich to win that war. Stalin never believed that the Third Reich would be an ally, and the attempt of people to spin it that way is so intellectually dishonest it boggles the mind.
You can say whatever you want. The fact is the Soviets had a treaty with Nazi Germany and only fought back when forced

They treated with the Nazis precisely because it postponed the threat.

And as freagle said, they tried to fight the Nazis first but couldn’t do so without western cooperation, which was refused. Delaying until ready to fight alone was the second best option.

Then why did they not end the treaty when UK entered the war but when the Nazis invaded the ussr?

Stop. That question assumes way too much. So let’s unpack it and reverse it on you.

Are you saying that ending the treaty when the UK entered the war would indicate to you that the USSR was a righteous ally of good and true humanity but that not ending the treat at that time would indicate to you that they were actually Nazi collaborators or at best willing to let the Nazis take over the entire world as long as they didn’t get attacked (which they knew would happen because again, the Third Reich was abundantly clear that destroying the USSR was its number 1 goal)?

Because if you can think through the answer to that, we can answer your question, which is that the USSR had always known that the UK was not ally, and it knew that because the UK and its allies, including the US, invaded Russia after WW1 to try to stop the communists from forming the USSR. The USSR, however, didn’t think the UK would be so evil as to literally turn a blind eye, and even financially support the fascists.

After the USSR sought to ally with the West to defeat the Nazis and they said “nah”, even you can see that the USSR was completely on its own to survive and the UK entering the war, while the majority of Nazi forces were on the Eastern front, did not change the strategic landscape enough to make the USSR capable of surviving an open conflict with the Third Reich in the fall of 1939.

Between 1939 (UK declaring) and 1941 (USSR declaring), the Red Army quadrupled in size. The idea that the USSR should have just decided to fight in 1939, when it was 25% the size it was when it eventually won is literally the same idea as the USSR should have lost the war but done so while adhering to your definition of morally good. It’s daft.