DeathEdit

• Boone was going to say, "Tell Shannon that I love her" but he died before he could finish it.
• As Boone was the sacrifice the island demanded, he and the island became one with his death and burial. When Locke had a vision of Boone in the sweat lodge in "Further Instructions" he was in fact talking to island which used the shape of Boone to communicate with Locke. The island might continue to interact with the survivors in future episodes in this way.
• If Boone didn't die, Locke would not have broken down screaming on the top of the Hatch. It is later shown that this act gives Desmond hope and stops him from ending his own life. Boone's purpose in death was to save Desmond and the rest of the Island inhabitants and perhaps the world.
• When Boone visited Locke in "Further Instructions", it was a manifestation of the monster.
• Died because he was meant to be in first class (the front section). That would comply with Shannon's death too.
• Boone was literally a sacrifice the Island demanded. In Locke's dream he saw Boone bloodied and saying "Teresa falls up the stairs, Theresa falls down the stairs." That is a phrase only Boone would know about. The phrase came to Locke in his dream, The Island sent him this phrase, because he would need to say it to Boone in order to convince him that he should come with Locke to look for the plane. Boone would be mystified as to why Locke knew about Teresa, and then decide to trust him and help him. The fact that Boone was bloody in the dream was the Island telling/hinting to Locke, that Boone would need to die. So in short, the Island sent Locke that dream as a way to lure Boone to his death, a sacrifice the Island demanded.

• The Man in Black sent Locke the dream as a way to indirectly lead Boone to his death, thereby killing one of Jacob's candidates. By extension, this would have also lead to Locke's death at Shannon's hands due to her grief over Boone's death, but Sayid prevented this.