Fellowship of the Ring page 414...
"I shall go to Minas Tirith, alone if need be, for it is my duty," said Boromir; and after than he was silent for a while, sitting with his eyes fixed on Frodo, as if was trying to read the Halfling's thoughts. At length, he spoke again, softly, as if he was debating with himself. "If you wish only to destroy the Ring," he said, "then there is little use in war and weapons; and the Men of Minas Tirith cannot help. But if you wish to destroy the armed might of the Dark Lord, then it is folly to go without force in his domain, and folly to throw away." He paused suddenly, as if he had become aware that he was speaking his thoughts aloud. "It would be folly to throw lives away, I mean," he eneded. "It is a choice between defending a strong place and walking openly into the arms of death. At least, that is how I see it."
Frodo caught something new and strange in Boromir's glance, and he looked hard at him. Plainly Boromir's thought was different than his final words. It would be folly to throw away: what? The Ring of Power? He has said something like this at the Council, but then accepted the correction of Elrond.
Let's go to the Council of Elrond and see what Frodo is referring to. Page 300.
Boromir stirred, and Frodo looked at him. He was fingering his great horn and frowning. At length he spoke.
"I do not understand all this," he said. "Saruman is a traitor, but did he not have a glimpse of wisdom? Why do you speak ever of hiding and destroying? Why should we not think that the Great Ring has come into our hands to serve us in the very hour of need? Wielding it, the Free Lords of the Free may surely defeat the Enemy. That is what he fears most, I deem.
The Men of Gondor are valiant, and they will never submit but they may be beaten down. Valour needs first strength, and then a weapon. Let the Ring be your weapon, if it has such power as you say. Take it and go forth to victory."
Elrond then has a big long speech and ends with I will not take the Ring to wield it.
"Nor I," said Gandalf.
Boromir looked at them doubtfully, but he bowed his head. "So be it..."
Now, I don't think for a minute he gave up his purpose just because Elrond and Gandalf said the ring can't be wielded. What if Boromir held Elves and Wizards in the same esteem his father did? He gave in it seems, but on page 414, he still has the same strategy.
I do believe that Boromir was influenced by the Ring, but I do not believe that he was suddenly overcome. I believe, he came to Rivendell on a desperate mission: to find out what Isildur's bane was. He did not know it was the Ring of Power. Faramir said that Boromir thought it was a great weapon. His mission was to bring that weapon to Gondor to aid in the defense of Gondor and bring glory to himself.
So, when faced with returning home without it... he tried to take it from Frodo.
What say you...
What say you..."
Boromir is man overcome by sin. But he is not wholly Caine. There is something of Peter in him as well, in that he is used for good following his evil act. And there is a bit of the thief on the Cross as well--the death-bed understanding of how he has erred.