Desmond's Dumb Smile by Marion

Des has that dumb smile and happily goes off with Sayid because he is fully conscious in both realities. He sees Sayid snap the other guy's neck and he knows that even if Sayid kills him or someone else does, death will not only be a quick trip back to Penny, but it'll be a quick trip back to the best moment of his life in ANY reality -- the moment when he meets Penny. Imagine you've just woken up from the best dream, and here's an opportunity to dream more. Or it's as Charley described imagining Claire before he was brought back to life. Des has no reason to fear death. I don't even think he's concerned that there's no little Charley in the Sidewaysverse because he probably thinks there'd be a course correction and little Charley would come along soon enough.

This would be a logical train of thought. Given that he is now stuck once again on Craphole and last time it took years to get off Craphole, death and a quick trip back to the Sidewaysverse may seem like the best alternative and shot at happily ever after.

I don't think he's trying to destroy the Sidewaysverse or bring the timelines together. He's just trying to get back to Penny and his plan is to show the other 815 passengers the wonderful reality he's found. Frankly, from his POV, the sooner he "dies," the sooner he wakes up from the reality he now finds himself in (being the prisoner of the father in law from hell.

The difference between Charley and Des in this is Charley isn't even looking for Clare. He's dead inside, a hopeless junky. Charley was actually better off on the Island. He kicked junk, met a great girl, became a father to her kid. The Island kept Des from his true love and fatherhood. For Des, nothing could be worse than being back.