The Council reveals why he is so blind in his understanding of the ring and why he would think later on that taking it was the only way to save Gondor, and Denethor and Faramir explain more of his character as to why he would be seduced by the ring in such a way, Poor Boromir! He has no clue what he's up against! Now the Ring is working on Boromir as well as Frodo. At this time Frodo has been fighting the Ring ever since Gandalf threw it in the fire. The Ring has tried time and again to betray Frodo to the Ringwraiths and nearly succeeded.
Boromir doesn't even know the Ring has already affected him.
My take on Boromir is that his very first instinct was to take and use the ring. He could clearly see that the entire council disagreed with him, and said no more about it, agreeing, in a sense, to follow their direction. But the "rightness" of destroying the ring never sat well with him. He was never convinced in his heart that destroying it was the right and best thing to do, but since he was "outvoted" on it, he went along with the council decision. I don't think he ever abandoned his misgivings about their mission, he merely stopped talking about it. I don't even really see it as an evil lust for personal power of the Ring. I think he just really believed first, that it could be useful as a weapon FOR Gondor, and second, that any attempt to enter Mordor with it would fail and they would be caught.
This is something we have all done. Usually saying "whatever you think is best" when we really mean, "OK, I think this is a dumb idea, but I will go along with the group rather than argue about it. When this fails, we will do it my way."
That's my take anyhow. I don't think he had a pre-meditated plan at that point, just a bad feeling about the mission.