Why was Stalin going to eventually turn on Hitler?
A question I often want to ask--was about to again in this thread but you beat me to it!

It's apparently an article of faith among so many here that
of course Stalin is just as evil as Hitler and therefore he
must sooner or later go on a world-conquering spree.
Cryptic above gives some good reasons why, aside from being a cartoon villain, Stalin would, one would suppose, surely attack eventually. An even better reason is that Stalin is the leader of a world revolutionary movement dedicated to the overthrow of capitalism and the dictatorship of the proletariat globally. This is supposed to be why the USSR exists. And whether Soviet leaders actually are fire-eating revolutionaries or not, their reputation as such must scare a lot of Western bourgeois regimes.
Now actually, I think this conventional wisdom around here is dead wrong--Stalin would never have launched an attack on a major power. And it's not because he was a good man at heart or anything like that. Nor is it merely that actual Bolshevik doctrine was not at all about conquering the world by the armed force of any nation, even one formed to be the bastion of world revolution--ultimately Marxist revolution is supposed to be a do-it-yourself sort of thing. That's true too, but that's not why Stalin would never attack.
He wouldn't attack for reasons related to why none of the revolutionary movements he got control of through Third International channels ever accomplished much. Because in general, Leninist leaders liked to play it safe, and in particular Stalin defined "playing it safe" as whatever kept
himself in charge.
So--Stalin did indeed devote a lot of the Soviet Union's resources to a huge military buildup, one that had no rational purpose, one would think, but to conquer Europe with. (Well, it turned out that the Soviets needed all that and more just to hold back a German onslaught. Who knew?) And then, just as things seemed to be shaping up nicely, he purged the military leadership. He purged the inventors, engineers, and technicians who had been building his war machine for him. Why? Because any organized force competent enough to take on the challenge of invading Europe is going to also have the capability of considering a coup against
him.
That's what I figure would be going on if no one attacked Russia for the next 14 years after 1940, assuming Stalin dies at the same time as OTL. He'd get everything lined up, then sweep it off the board and start over. He'd procrastinate and never get around to actually launching the attack.
OTOH if someone attacked him--well, as OTL, he might be taken by surprise and shocked. But after a a week or so of drunken funk (per OTL) he'd come back to work, take control back from Molotov (or whoever he'd eventually replace him--OTL Molotov did survive Stalin and eventually died a natural death in the 1960s so perhaps it would always be Molotov) and find that despite the general devastation the enemy inflicted, he still had lots of force left, and he'd start deploying it. (Actually Molotov already did that for him, but the Boss gets the credit, and can double down rather more decisively too).
Then the Soviet steamroller finally gets going.
...The strain on Britain would've become serious enough without the entry of the USSR or USA that I can't see how they avoid having to at least enter negotiations after a while.
But only if the Germans continue the war will Britain be under strain. If the new German leadership, whoever they may be, doesn't make substantial changes in their policy immediately, everyone will continue on the assumption that they are essentially interchangeable with Hitler.
If the successor regime does not launch Barbarossa, what exactly will they do with all their accumulating might? They've already conquered Europe; they can't reach Britain. How do they apply strain to Britain?
Mainly by submarine warfare, it seems, and an ongoing Blitz. These are the kinds of excuses FDR needs to continue his "Germany first" policies.
The German leadership would probably be thrown off balance by Hitler's many impressive victories, plus 7 or more years of Nazi propaganda, plus their own inherent arrogance, and judge they need to play the hand that Hitler dealt.
And like another poster mentioned, what if Hess had started an openly CONCILIATORY policy toward the U.S., with no declaration of war after Pearl Harbor? How does FDR get a "Defeat Germany first" strategy with no overtly hostile Nazi leader and no second front in the form of USSR military involvement? They'd likely concentrate on the Pacific War and that's it.
OK, now you're confusing me. Is Hess or whoever at war with Britain, or not at this point?
If he's not, the "Defeat Germany first" platform is irrelevant; now there is no war to win.
If he is--he's still trying to cut Britain from world trade, so his protests of Japan's deplorable attacks on the USA ring a bit hollow.