If the characters are going to die, I think it should have been handled better. Personally, I don't think Newt should have died, but I don't think she should carry on the torch of battling Aliens. It's already scary enough that she witnessed the gruesome deaths of her parents, sibling, and several of the Marines who'd saved her at such a young age. Going home and dealing with serious PTSD is not a happy ending.
The same can be said for Hicks. I wouldn't mind seeing more of his character and how he struggles to deal with the loss of his unit, of people he got close to since enlistment. Does he even want to continue serving after that? Would his mental health even permit that? Those are the stories I would've liked to see instead.