Upgrade by Blake Crouch. Spoilers below.

>!The main character's mom caused a global famine with her gene editing technology by accident due to unintended side effects... And the main character's goal through the book is to prevent somebody else using the same technology to edit genes because he's worried about the unintended side effects that they can't predict...!<

>!But when he saves the day and stops them, he takes the technology and uses it to edit the entire global population himself! But wait, it's okay because he only edited people to be more empathetic, so he doesn't have to worry about any unintended side effects! Because he had good intentions!!<

When I think of bad endings to books I've read, this will always be at the top of my list. And I've read a lot of Stephen King.