Original: YouTube.com
The way it is, is that there is no apocalypse and no monsters. It's all a suicide.
The girl on her deathbed was someone close to him and I'm guessing he had a mental breakdown. That why the Blue and Red endings are so similar. He kills himself. In the Green ending, he was able to let go. The reason he was seeing her everywhere was because she was on his mind 24/7 and you start to see only them over and over.