The father ?
I've just finished rewatching the movie for the second time with my brother and realized something about the last appearance of the thing that's been following the protagonist : it took her father's shape whom I guess had passed away long ago. It struck me in a scene right after when there's a close-up on a family picture in a frame.
That's the reason why Jay didn't want to tell her sister what the creature looked like during the swimming pool scene.
I'm sure there's a link between the father's assumed death and the blood in the swimming pool but I can't put my finger on it... does anyone have a clue ?
It's hard to understand if they actually killed the thing or not... did she have sex with Paul because she wanted to be sure the thing was gone or was it a metaphore implying she finally had sex with the "right" guy (which was never the case before) ?