In Jason's defense: he never dreamed that Elizabeth would end up not leaving Collinwood for 18 years. If he had known, I don't think it would have changed his actions - but still, he didn't know.
And I can't quite see Elizabeth as completely innocent in the matter. She did hit Paul with the poker - at least, I think it was a poker - and she could have killed him. And what was she accomplishing by not leaving the estate? As has already been pointed out, she wasn't going to hear anybody breaking and entering the locked room in the basement if she was outside the house.
Anyway, if one has to spend one's whole life in one place, Collinwood isn't such a bad place. Nice woods, a beach, ghosts to tell you bedtime stories...not bad at all.