[SPOILERS] The problem with the ending isn't that it's a downer...
...but rather that the wrong sibling died.
Had Tim been the one to get killed the device, leaving Kaylie to not only get blamed for her brother's death, but also the death of her fiance, the film would've become an absolute masterpiece of modern horror.
Think about it. The girl who seemingly led a normal life after the her parents' deaths and her brother's institutionalization, suddenly snaps after reuniting with her recently released brother, would've been quite the profound closing statement.
Instead, the ending goes for some lame "repeating the cycle" garbage that weakens the final product a little. Despite that idiocy, the film is still very excellent and chilling enough for me to enjoy it immensely.
I just personally think one vital change would've made the film absolutely perfect for me on every level.
"What's the ugliest part of your body? I think it's your mind."