While I agree with you that Ukraine vs. Putin is clear-cut, morally speaking, I don't think we can apply pre-Atomic Age logic to the situation. I just watched this scene in Crimson Tide:
XO Hunter: "...I just think that in the nuclear world, the true enemy can't be destroyed."
Captain Ramsey: "Attention on deck. Von Clausewitz will now tell us exactly who the real enemy is. Von?"
XO Hunter: "In my humble opinion... in the nuclear world, the true enemy is war itself."
I think that's the difference. In WWII, appeasing Hitler was a mistake because it only emboldened him. Even back then, Tolkien had reservations about how far to take the war. While he abhorred Hitler, he initially supported Neville Chamberlain's "appeasement" policy.
But today, a nuclear Russia cannot be defeated the way Germany was. "In the nuclear world, the true enemy is war itself."
We cannot allow a nuclear war to happen, even if it means letting Putin have Ukraine.
The question everyone is wrestling with is, how do we save Ukraine while preventing nuclear war? Unfortunately, we don't know the answer, and the risk of getting it wrong is catastrophic.