OUT OF UNIVERSE ANSWER
Tolkien said that using the Ring to defeat Sauron would be inherently evil. Commenting on the war effort late in WWII, in a letter dated 1944, he said:
An ultimately evil job. For we are attempting to conquer Sauron with the Ring. And we shall (it seems) succeed. But the penalty is, as you will know, to breed new Saurons, and slowly turn Men and Elves into Orcs. Not that in real life things are as clear cut as in a story, and we started out with a great many Orcs on our side.
-Tolkien, Letter #66.
Obviously, he wanted the Allies to win, but he disapproved of how the Allies were going about it.
He also discussed what would happen if one of the most powerful (and most 'good') characters in The Lord of the Rings used the Ring to defeat Sauron (spoiler alert: it wouldn't be good):
Of the others only Gandalf might be expected to master him – being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form. In the ‘Mirror of Galadriel’, 1381, it appears that Galadriel conceived of herself as capable of wielding the Ring and supplanting the Dark Lord. If so, so also were the other guardians of the Three, especially Elrond. But this is another matter.
It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power. But this the Great had well considered and had rejected, as is seen in Elrond’s words at the Council. Galadriel’s rejection of the temptation was founded upon previous thought and resolve.
In any case Elrond or Galadriel would have proceeded in the policy now adopted by Sauron: they would have built up an empire with great and absolutely subservient generals and armies and engines of war, until they could challenge Sauron and destroy him by force. Confrontation of Sauron alone, unaided, self to self was not contemplated. One can imagine the scene in which Gandalf, say, was placed in such a position. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors.
If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been destroyed, taken from him for ever. But the Ring and all its works would have endured. It would have been the master in the end...
Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. He would have continued to rule and order things for 'good', and the benefit of his subjects according to his wisdom (which was and would have remained great).
[The draft ends here. In the margin Tolkien wrote: 'Thus while Sauron multiplied [illegible word] evil, he left "good" clearly distinguishable from it. Gandalf would have made good detestable and seem evil.']
-Tolkien, Letter #246
The Ring was inherently evil, and even if you used it to do good things, the good things would all be evil when you get right down to it.
And so now Galadriel's outburst makes sense:
And now at last it comes. You will give me the Ring freely! In place of the Dark Lord you will set up a Queen. And I shall not be dark, but beautiful and terrible as the Morning and the Night! Fair as the Sea and the Sun and the Snow upon the Mountain! Dreadful as the Storm and the Lightning! Stronger than the foundations of the earth. All shall love me and despair!
- The Fellowship of the Ring, Book II, Chapter 7: "The Mirror of Galadriel".
As for Boromir, Gandalf addresses this point later, when Denethor says Boromir should have brought the Ring to him.
"In no case would Boromir have brought it to you. He is dead, and died well; may he sleep in peace! Yet you deceive yourself. He would have stretched out his hand to this thing, and taking it he would have fallen. He would have kept it for his own, and when he returned you would not have known your son".
-The Lord of the Rings, The Return of the King, Book V, Chapter 4: "The Siege of Gondor"
But Gandalf already mentioned this at the Council of Elrond:
Its strength, Boromir, is too great for anyone to wield at will, save only those who have already a great power of their own. But for them it holds an even deadlier peril. The very desire of it corrupts the heart. Consider Saruman. If any of the Wise should with this Ring overthrow the Lord of Mordor, using his own arts, he would then set himself on Sauron's throne, and yet another Dark Lord would appear.
-The Lord of the Rings, The Fellowship of the Ring, Book II, Chapter 2, "The Council of Elrond"