The story Foster tells to the family after he crashes their barbecue, that ends with him and his family "all sleeping together in the dark", which seems to imply that Foster intends to kill his wife, his daughter, and himself once he gets to them. The dreamy way he tells the story just makes it worse, as he genuinely seems to feel that is the right thing to do.
The whole movie becomes this once you get to the end, in part due to Paranoia Fuel. Given that many people find Foster sympathetic, it makes one wonder just how easily this could actually happen to them or someone they know.
The Whammy Burger scene can be this for some, considering that this could happen to anyone.