In any case, the adjective dead shows the participial form that so many English adjectives are constructed from. In origin, dead is the old way to say "died." Likewise, "death" shows the -th ending that we use in English for so many nouns describing a quality: warmth (the condition of being warm), tilth (ground being tilled, that is, farmed), strength (the quality of being strong), death (the condition of being dead).
Underneath our bloated vocabulary, stolen from every language on earth, English retains its Germanic structure. The basic building blocks of our language go back over sixteen hundred years to the cluster of mutually intelligible dialects spoken by the Angles, Saxons, Jutes, and Frisians.