"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.

Somehow English "for good" reminds me of German endgültig which also has the meaning of "forever". But I think it would be quite a task to show that there might be a relationship between the German word element -gültig and English good. It would be necessary to see whether there are old variants of modern German endgültig in Old Saxon, Old Low Franconian/Frankish (Dutch), Old Frisian etc. An old variant of -gültig might have become similar to good and then been replaced by "good". But I don't think that it is possible to verify such an evolution. The written instances necessary for verifying such an evolution are too sparse.