system
Given the following post, respond as if you're commenting on Reddit:
human
[Hancock] If Mary wanted Hancock to survive and be a hero that the Earth needed, why didn’t she just die and let him be sole survivor?

Watched the movie and it was pretty cool. Their weakness was kind of dumb though. So why not just die and let Hancock be the only survivor?
gpt
Because they're inexorably tied together.  If she dies he dies.  The longer and closer they are too each other the weaker they become until they're mortal but if somehow one were to die the other would die at the same time.