I have a theory that is far too crazy, so you shoudl better forget it again right after you read it ...
So ... maybe ... it's not about the Winchesters, but just about the women ... I mean ... I'm still at our part of the theory where a member of one certain family (most likely a female member) did something bad ...
And now there's a ghosts, demon, whatever tracking down the family and killing all females ... maybe only all members of that family who have blonde curled hair, I don't know ...
So, now think about it ... if whatever bad thing happened happened a very long time ago, which I think is the case ... then the family will now be spread all over the country ... maybe even the world ... and there are a lot of strangers, who have the same ancestors, but - of course - don't know about it.
So maybe Jess and John't wife had the same ancestors ... a looooooong time ago ... and that's the reason they had to die ...
I mean, it's TV ... I think almost everyone thinks that it has to do with the Winchesters, since it killed first John's wife and then Sam's girlfriend. But maybe it's not about them ... maybe that's just (TV)coincidence.
I know, crazy ... that's what this show is doing to me ...
Something about your theory
Sonya:
Maybe he doesn't kill them because he wants to protect the men of the Winchester family, but because he's simple jealous ...
I mean, think about it ... he had a wife who he loved, who he adored ... and she was pregnant ... he was the happiest man in the world. Beautiful wife who was bearing their beautiful child ... she loved him, he loved her.
So, now ... for some reason ... she kills herself ... and I would guess that the woman who killed herself wasn't pregant, bec then this thing would kill pregnant women. So she kills herself for some reason and he becomes sad. Really sad and angry, maybe doesn't care for the child that much anymore, because all he can think about is that his beautiful wife killed herself. It was a perfect family for him and now it's a tragedy ... something he can't live with. It takes everything of him ... the thing he loved the most and his oerfect life. He couldn't be ahppy anymore. He still had their child, sure, but everytime he looked at him / her ... he saw his wife ... who turned into the person he hated the most 'cause she destroied thier happy life ... his happy life ...
And now there are the next generations of Winchester men ... getting married, having children and / or being simple happy ... happy with their lives and families ... and this man who had a happy life a long time ago and then had to face pure sadness and anger ... he gets mad ... really mad ... and jealous ... 'cause all those other men live the happy life he once and had and was dreaming of having forever. But he couldn't. So he doesn't want anyone to have it. At least no-one from his family ... so he kills their wives / girlfriends when they are as happy as he's been and are about to be even happier ... that thought kills him (ok, forget about the fact that he ... it ... whatever is already dead
).
So he takes that away from the Winchester men ...
And to fix that, Dean, Sam and John (or whoever) has to find his wife ... her ghost that is (of course, like every ghost lol) still somewhere too far away to see, but close enough to somehow reach ... with a spell, probably ...
So, if they find that woman they have to bring him to her ... bring them together again ... with the ghost of their child ... and show him that he can be happy - again ... he still has his family and he'll never lose it again ... they'll be together and happy forever ... just has he wanted.
(in that case, the woman probably died bec of a desease and thw whole village thought she killed herself ... with poison or whatever ... 'cause otherwise she'd have to explain why she killed herself
)