Why was the devil given power over death? From Hebrews 2:14-15,


  Since the children have flesh and blood, he too shared in their
  humanity so that by his death he might break the power of him who
  holds the power of death - that is, the devil - and free those who all
  their lives were held in slavery by their fear of death.


I'm sure I've seen this mentioned elsewhere in scripture as well, but what I don't recall is why. Is it ever stated why Satan was given power or control over the death of humans?
Goodness exists.
Evil is not an object, it exists only as the negation of Good. Evil cannot create, it is a name for destruction. For instance, punching a stranger is bad because it would negate his happiness and his comfort (somewhat!)...but neither the fist, the motion, nor his face exist as evil objects. (Augustus felt this way- and the very great Mr CS Lewis agreed).
The sum of all goodness is in God.
Lucifer rebelled against God, so rebelled against goodness, so rebelled against creation. His is none-creation. His, therefore, is death. All who follow Satan follow true death because they have removed themselves from 'being'. All who follow God die to be raised in a perfected creation of goodness.