Ehem... Dumbledore was not near death... since when could a near-death person ride a broom or cast a spell? Dumbledore knew that if Malfoy didn't get the job done, Malfoy would get killed. Dumbledore is optimistic, and thus, he probably believed that Malfoy would do something great, and he traded his life for Malfoy's. The argument in the forest was probably about that, as somebody else posted in another thread.