The Pulpit Commentary on Genesis 21:15

And the water was spent in (literally, from) the bottle,—so that the wanderers became exhausted, and were in danger of fainting through thirst—and she cast the child—a translation which certainly conveys an erroneous impression, first of Ishmael, who was not an infant, but a grown lad (vide supra, Genesis 21:14), and secondly of Ishmael's mother, whom it represents as acting with violence, if not with inhumanity; whereas the sense probably is that, having, as long as her rapidly diminishing strength permitted, supported her fainting son, she at length suddenly, through feebleness, released his nerveless hand as he fell, and in despair, finding herself unable to give him further assistance, left him, as she believed, to die where he had flung himself in his intolerable anguish—under one of the shrubs.