What happens at the end of Paranormal Activity?

Answer 1

Catie go's downstairs and yells "MICAH!!!!!!!!!!!!" and he, Micah, jumps out of bed and goes downstairs and Catie kills him. Then she goes upstairs and roars at the screen and whacks the camera.

Answer 2

Sometime in the afternoon before the last night, the demon takes possession of Katie again, and, pretending to be Katie, it convinces Micah to stay in the house one more night (remember when she looks at the camera with that really creepy look and says, "I think we'll be just fine now." That's the demon talking). Then in the middle of the night, the demon makes Katie get up, walk down the stairs, and start screaming, in order to lure Micah downstairs. When Micah runs down to help, presumably Katie stabs him to death with the huge kitchen knife we saw him using (I think to chop vegetables) in the beginning.

Then she brings his body upstairs and hurls it at the camera. Then she hovers over the body and kind of sniffs it like an animal, then comes at the camera and stares at it, again, with a creepy smile on her face. Then her face becomes a demon face right before the credits come.