Death, decease, demise, passing denote the end or the ending of life.
Death is the general word and is used for the termination of plants and animals as well as of men and also of inanimate things marked by continuity or development.
Decease and demise apply only to human beings except in figurative use. Decease is preferred in legal context but in ordinary use conveys a slightly euphemistic or rhetorical quality. Demise in literal use has become somewhat pompous or affected.
In figurative use it frequently lacks such connotation.
Passing is a euphemism for the death of a person.