**SPOILER** Why do YOU think Joel chose to spare Ellie rather than use her to find a cure?

I think after the whole adventure, Joel had become attached to Ellie. After bonding with her throughout the whole experience, he kind of accepted her as a second daughter and he didn't want his "second daughter" to be killed. So even if it means humanity is doomed, he wants Ellie in his life and would do anything to ensure her safety

He saw her as another daughter in his eyes, he didnt want her to die like sarah, so he saved her

It was mainly because they had been through so much together. I guess Joel also kinda missed his little girl,
(Sarah) so he wanted to fill that void in his heart with Ellie :3

Moreover Joel didn`t saw a reason to give mankind a second chance, because mankind killed his daughter. I think he believes, a society that is as cruel and immoral as these isn`t worth to linger. Mankind doesn`t seem to have a sense for morals anymore, so why shouldn`t he act igoistic as well? That`s why he would never let Ellie die for the sake of mankind.

cause he obviously loved her like another sara