The JW's use "undeserved kindness" because Freddie Franz was too anti-historical-Christianity to use the term "grace".
grace (in Christian belief) = "the free and unmerited favor of God, as manifested in the salvation of sinners and the bestowal of blessings."
Ask just about any Christian and they'll be able to explain what "grace" is then ask a JW what is "undeserved kindness" and you'll get anything but an official Watchtower definition because the WT has a habit of redefining words to suit the hierarchy of their sectarian/religious-cult organization.