Is there a word for someone whose life was possible because of another's death?
The closest I can think of is : step into dead men's shoes though that may imply also taking on the role of the dead person.
To take over the position or role of someone who has died.
Derived from the proverb, "It's ill waiting for dead men's shoes."
With the economy as stagnant as it is, the only way to get a promotion these days is to step into dead men's shoes.
- http://idioms.thefreedictionary.com/step+into+a+dead+man%27s+shoes