Killing Elend and leaving Vin alive would have been, in my opinion, more tragic than what happened. As I establish in a little bit, there is an afterlife in this cosmology. Better for them both to die and to be together.
There were only two ways that Ruin could have died in this book. The first would be to have him give up his life as Preservation did. I don’t think that was very likely.
The second way is the one I’ve been subtly pushing the reader toward from the very beginning of the novel. Ruin and Preservation are opposites. Equal, particularly while Ruin doesn’t have access to the chunk of his power trapped in the atium. The only way, then, for him to be killed would be for Preservation to smash his power against that of Ruin and destroy both of them. It’s a form of balance. Either you block and stop each other, warding each other away, or you overlap and destroy one another.
This was the role Preservation chose Vin to play all those years ago. As she surmises, he needed someone to do what he could not. He had been too corrupted by his power, and could not destroy Ruin. If Vin had held the power for millennia as Preservation had before her, then she too would have lost the ability to destroy Ruin.
It needed to be someone fresh to the power, still separate enough from it to be able to kill Ruin. Preservation knew that if he did not sacrifice himself and let someone else take up the power, then Ruin would eventually win and the world would end. Imprisoning Ruin was always only intended to be a delaying tactic.
The delay was so that the power could find a new person to bear it. Someone who could do what Preservation could not.