Shlomo HaMelech writes in Mishlei (Proverbs): "The name of the wicked should rot (& not be remembered)." The Gemara (Yoma 38) says this means that one should not name their child after a rasha, an evil person (e.g. Esav, Nimrod), because this may negatively affect the child’s character. So why, then, do we find a famous rabbi named Yishmael (whose teaching we read each day in the morning Shacharit Tefila), as well as the pious Kohen Gadol in the 2nd Bet HaMikdash who was one of the 10 martyrs killed by the Romans? After all, Yishmael was expelled by Sara & Avraham for his sinful behavior! Some say that since Yishmael eventually repented (see Bava Batra 16b) his name is allowed. Others (e.g. the Ritva) point to the fact that this name (like that of Yitzchak) was chosen by Hashem himself prior to the birth & has the beautiful meaning, "Hashem should hear," & so is permissible for a parent to use. Nevertheless, it is rare indeed to find a child today named Yishmael.