"for good" expression in an unfortunate event?

For good simply means forever or for good and all.

• forever, permanently forever; permanently
  I finally left home for good.
  They tried to repair it many times before they fixed it for good.

• for good (British, American & Australian informal) also for good and all
  (American informal) forever
  I'm leaving for good this time.

• permanently or forever
  Now she says she's leaving him for good.

This of course differs from for good as opposed to for evil.

I always understood the full expression to be 'For good or ill'. In other words, regardless of his, her or its fortune. Now often the word 'good' is a mispronunciation of 'God' (as in good-bye - God be with ye). That adds quite a different slant on the subject.