"Then as it was, then again it will be" - simultaneous with "A shot right out of the blue" If it were anyone else it would reek to high heaven of a trap in the making, and he's given Ellen reason enough for springing one to not even begin to count as a betrayal. Bringing the boys along is just asking for trouble if someone's gunning for revenge, someone with a score to settle and a broken family left in the Winchester wake. John's been in this game long enough; he can read the signs as easily as witches read cards or tea leaves or entrails or whatever the fad of the week is.
Which, in the end, is why he goes: there's none of the markers he'd expect, and nor is there the studied absence of them that would upgrade it from reeking to screaming 'Bad Plan'. There's just Ellen, sounding careworn but fiery as ever, and news about... Sam? And concern, the sort only a parent really 'gets', and he can't not investigate, not when he has his own suspicions. He'd feared it would come to this sooner or later – other hunters aren't exactly known for their tolerance, and demons do like to shit stir – and... well, if it is to be Showdown At The Okay Corral (or the Harvelle Roadhouse) then better sooner rather than later, right? Strike while the iron is hot.
And it's everything he expects, and every bit as awkward (when did the girl get so big? It doesn't feel like seven years). Now is not the time for explanations, though, not when there's business to be done.
Ellen takes him aside, leads him through the kitchen (there's ghosts in this place, and not the sort you can get rid of with salt and fire – Bill at the stove, Joanna Beth barely knee-high to a grasshopper rummaging for her piece of candy and Ellen saying she'll grow up spoiled, John arguing that little girls are supposed to be, right?) upstairs into the living quarters.
(He never brought candy back for Sam or Dean - fat children can't run away from monsters. Brittle bones won't survive a fall, or stand up to a beating. Rotten teeth don't stand up to chewing through restraints - but he never gave Jo a gun either).
There's things he should say – things like the way things ended was shit, Bill deserved better, and I should have come sooner, and meaningless compliments about how well she raised Jo and a million variants on I'm sorry... but John, of course, is all business, noting where the obvious weapons are along with the most logical locations for concealed ones and planning at least three escape routes (and doing it fresh, because while he used to know this place like the back of his hand complacency - assuming you'll be okay rather than taking active steps to ensure your survival - will get a man killed quicker than any monster). “So, this hunter friend of yours.”