what happened to kat?

i'm sure this has already been posted but i can't find it in recent posts - we know kat died, but how? and when? what did clark say that might have given clue into how it happened?

i remember when the fly went through the wall, but then about 2 minutes later we saw it was flying over someone's body in the corner of the screen when mary was walking through the backrooms. im assuming that was kat, no longer with a head. i feel like everybody but me missed that for some reason. it was fast, but still.

didn't clark say "i did everything i could to help her". what does that mean?? did they band up and hunt her down? did she lash out at him for causing bobby's death, and for putting her in the backrooms to die? what was the conversation they had in the pool area about? how could she see him and him not see her? did it have to do with mary's book about the window?

too many thoughts. help me out plz