Boone was going to say, "Tell Shannon that I love her" but he died before he could finish it.
Before he died, Boone wanted to tell Shannon that he didn't need her anymore. This is because in the episode "Hearts and Minds", Boone experienced a hallucination and admitted that he felt relieved when Shannon died, meaning that he had moved on.
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.
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.