“And God will give you mercy, and show mercy to you” (Deuteronomy 13:18). God will instill in you the trait of mercy and compassion; then He will “show mercy to you.” If one has mercy upon living creatures, Heaven will have mercy upon him (Shabbos 151b). However, if a person lacks mercy, there is no difference between him and a beast, which is not sensitive to the suffering of other creatures (Rabbi Yehudah HeChassid, Sefer Chassidim, 87).
“One should respect all creatures, recognizing in them the greatness of the Creator Who formed man with wisdom. All creatures are imbued with the Creator’s wisdom, which itself makes them greatly deserving of honor. The Maker of All, the Wise One Who transcends everything, is associated with His creatures in having made them. If one were to disparage them, God forbid, this would reflect upon the honor of their Maker.” (Ramak, Tomer Devorah, ch. 2).