The bridge referred to is what gets you past the "great danger" as a bridge crossing a river. The concept is similar to the saying, "the enemy of my enemy is my friend"; once the first enemy is dealt with you need to reassess your friends. USA and USSR were friends until the great danger of the Third Reich had been dealt with; immediately after the two "friends" became hostile while the USA and UK remained allies.

How does all that relate to survival? Does it justify cannibalism to avoid starvation? That has happened.

I see the quote as something to rationalize doing something that is socially unacceptable -- way outside the norms and possibly illegal. Breaking into a Wal-Mart to get what you need to survive. Stealing a car to get away from danger when other means just won't work. Killing someone trying to get your stuff so that he can make it through another day.

Another way to look at it in a survival sense is that you are doing what is necessary in the moment because you didn't do what needed to be done to prepare before the moment.

I'll leave it there, but I'll probably be back.