Are you retarded?
Like the guy above me just said, "Tail Beast cannot die!"
First, Kishimoto only made it seem that Madara wanted to kill Bee, but in reality it was to make Naruto look a total showoff.
Second, Madara is not after Bee's body. He is looking for the prize money which is the chakra.
Third, Madara is a prodigy do you think that he would go for a fatal blow or even if he did go for it. He would just steal the chakra to deliver it to the Gedo or wait until he defeated Naruto to hand the Gedo both of their respective chakra sources.
Fourth, Madara might have of had a change of plan for all i know. He might not want them anymore and considers both of them a hindrance for the perfect world that he keeps mentioning every single second. (Although this fourth option is less likely to be true)