Ok, I know you all think it would be stupid for Dumbledore to plan his own death but what if he had no option. Snape had to make the Unbreakable with Narcissa to prove his loyalty to Voldemort. He then would have told Dumbledore about this (Snape is a good guy fo sho in my opinion). Dumbledore told him he must keep this Vow and should therefore kill him if it became nessecary. That is what him and Snape were arguing about. Dumbledore wasn't afraid to die and he knows that he must teach Harry all he can before he does (Horcruxes). He also knows that Snape can help Harry when the time comes. Snape is in an ideal position to protect Harry without Voldemort knowing. I feel Dumbledore sacrificed himself to save Harry, Snape and the wizarding world. Just a theory. Also Dumbledore would NEVER plead for his life in my opinion. I think he was pleading for Snape to kill him as planned and the look of hatred was because Snape hated himself for killing the one person who truly cared for him.