Why was Abraham selected to be the father of our people? Knowing of his kindness and his bringing the people to believe in one creator, yet he dismissed Hagar and their son from his camp with nothing more than their clothes on their back and some water . Surely this troubles our rabbis to this day.
Your question is on God, not on Abraham- for it was Abraham who did not (!) want to chase away Hagar and Yishmael (Gen. 21, “and it was very (!) bad in the eyes of Abraham”), but God nevertheless told him to do so, as He explains there explicitly, so that it should be clear to all that Isaac, and not Yishmael, is the sole continuation of the Chosen Nation. In addition, although the Torah is usually very concise when it comes to giving insignificant details, God does (!) feel it significant to write explicitly that Abraham himself (not through his servants) made sure to give them food (see there) and water, and apparently she was heading back to her father’s family (as was the custom for divorced women). The Torah also infers that their provisions were enough, but the problem is that they got lost and wandered [ibid, 21, 14-15, the Hebrew word “vateta”, is translated in the classic translations and commentaries (Targum Unkolos and Targum Yonatan, Rashbam, Ibn Ezra, Shadal, etc.): “and got lost”]) and only consequently of their getting lost, they ran out of water. This is also clear from the use of this root, in other places that it’s always “to leave the path” (Psalms 119, 110; Ezek, 14, 11; Job 15, 31, and 12, 24; Isai. 19, 14, and see also Baba Kamma 81a), or in our context, to get lost. It should also be pointed out that Hagar knew her way back to Egypt (see 16, 6 and on), but somehow here, she got lost. In general, may I strongly suggest learning Hebrew and then Aramaic, and studying the Scriptures as they were originally written, which will enable you to learn Torah and the classic commentaries, more accurately than from English translations. With Love of Israel, Rabbi Ari Shvat