"Please, Severus."

[Hannah]

Well really it's all explained in the last book, but Ddore was pleading with him so that his plan could be carried out, which was for Snape to kill him and get his wand, and to ensure the trust of Voldemort. Ddore was going to die eventually anyway because of the cursed ring. He knew that being killed the way he was would be much less painful than going through what he knew would be a long and painful end.