"Your Kasha Is Really Not A Kasha At All"
Rabbi Lazer Brody commenting on Bricks & Yishmael - Seeking To Resolve A Troubling Midrash
Your kasha is really not a kasha at all. Yishmoel was a non-Jew from a non-Jewish mother (Hagar), while the babies in Egypt were Jews. A non-Jew is not tried on midas ha'din - he gets it lenient in this world, and then pays heavy in the next world (See Avoda Zara 2a&b). "Ba'asher hu shom" is therefore not necessarily in the non-Jew's best interests, because Hashem is allowing him to accrue an enormous spiritual debt that he'll have to pay for.
On the other hand, Jews live tough lives so they won't have severe judgments in the future. Hashem preferred to let the babies die as martyrs, granting them eternal bliss in Gan Eden, rather than letting them grow up to be Avoda Zora junkies like Micha that deserve the death penalty a thousand times over. So, in the case of the babies in the mortar and cement, it was loving-kindness and ultimate compassion on Hashem's part. Hashem doesn't do the same favor for a Bin Ladin or an Achmedinejad.