The readings today sure have a lot of departures in them. Hagar departs with little Ishmael (whose only offense, besides being inconveniently born, was playing with his half-brother); the demons depart from the two possessed men; and Jesus departs from the Gadarene territory after his healings wreck the local economy and nobody really wants to see what might happen the next time he shows his mercy.
The readings in this Genesis cycle are a bi-ennial liturgical turn-off for me because Sarah comes across as so ugly, hateful even. This is especially clear in the case of Ishmael, who was sent away from his father because of the rather insane jealousy of Sarah. In trying to work through this, I found some interesting insights in the online Jewish Women's Archive; evidently one Jewish tradition sees that Sarah's jealousy, which leads to the dismissal of Hagar and her son from Abraham's home, turned out for Hagar's good, because she (much less her son) would never be able to flourish around Sarah. Plus, God takes the opportunity to offer Hagar his own word of promise about the boy.
Even though the circumstances are none too pretty, grace comes to the fore.