Do You Need to Tie Up All the Loose Ends to Have a Good Ending?

One of my favourite films is Predator.  There's something v satisfying about seeing muscle-bound Arnie having to use his brain rather than brawn to defeat the all conquering alien.  But the bit I really like is at the very end when Arnie has the alien at his mercy and is about to deliver the final blow, then pauses and asks, "What the heck are you?"

The alien, all quivery mouth tentacles and dripping green fluorescent blood, pauses then taps the question into his handy arm pad.  The pad flickers - it's translating the question.  And then the alien laughs.

He's about to die, and he laughs. Then he pyrotechnically explodes, and that's basically the end.

What makes it work for me?  Human intelligence beats alien technology?  Humans are essentially compassionate while aliens eat people?  Perhaps it is those things, but I think it's more that, while the alien is beaten physically, his spirit refuses to accept defeat, laughing in Arnie's broad and baffled face.

And as a writer I think, how brave not to explain who the alien is, and how he came into the jungle.  It is a mystery, and we will remain as baffled as Arnie.

With my first book, Adultery for Beginners, the original ending went a bit further to give a very obvious happy ever after ending but my editor stopped me.  'We don't have to know exactly what happens,' she said.  'It's enough to know that the main problem is over, and a new and happier story is starting for the main character.'  

We don't have to tick all the boxes and tie up every loose end. Just, solve the main story problem (the predatory alien is dead) and hint about the direction the main character is heading in (Arnie's going home). Most of the rest can be left to the reader's imagination.