Musar for Bava Metzia 169:6

To have mercy upon all of the creatures: He must also have his mercy extend to all the creatures. He [should] not disgrace them nor destroy them. As behold, the Highest Wisdom is spread over all the creatures - the inanimate, the growing (plants), the living (animals) and the speaking (people). And we are warned about disgracing food for this reason. And about this thing it is fitting that [just] like the Highest Wisdom does not disgrace anything in existence and everything was made from There, as it is written, "You made all of them with wisdom" - so [too, should] the mercy of a person be over all of His creations, may He be blessed. And for this reason was the holy Rebbe punished: Since he did not pity the young calf that was hiding with him and said to it, "Go, you were created for this," afflictions came upon him (Bava Metzia 85a). As they were from the side of judgement; since behold, mercy protects from judgement. And when he had mercy on a weasel and said, "It is written (Psalms 145:9), 'and His mercies are over all of His creatures,'" he was saved from the judgement - since the light of Wisdom was spread over him and the afflictions withdrew. And upon this way, he [should] not disgrace anything in existence from that which exists, as all of them are with Wisdom. And [so] he [should] not uproot a plant except for a need nor kill an animal except for a need. And [then] he [should] chose a nice death, with a checked knife, to have mercy in as much as is possible.