Why is the lifeless body referred to as better

I think you have misread this poem slightly. At the end of Longfellow's poem "The Slave's Dream," he does not say that the lifeless body is "better"—he describes it instead as a "worn-out fetter." The word "fetter" is more usually used as a plural, fetters, and generally refers to irons clamped around a prisoner's arms or legs. This makes it a particularly poignant word for Longfellow to use in this context, as he has been describing slaves who may have spent their whole lives in literal fetters or irons. Because of this, then, Longfellow is saying that their lifeless bodies are the "fetters," or constraints, in which their souls have been trapped for the duration of their time on earth. Once the soul leaves the body, the body is no longer a person, but simply a trap or container in which that soul has been anchored. The use of the word "fetter" to describe this abandoned body makes it clear that the soul contained within it did not enjoy its time there; on the contrary, in escaping that "fetter," it is removing itself from a life of slavery and imprisonment. As an unfettered spirit, it will experience a freedom in death which it did not have in life.