Use the adjective merciless to describe someone who acts in a cruel, heartless way. You could accuse your rabbit-hunting brother of being merciless.
Merciless is the antonym, or opposite, of "merciful." If a person shows no mercy or pity, she is merciless. This can describe a cruelly violent person, or it can be more figurative: "The torrential rain was merciless, pouring down on the wedding party as they ran to take cover." The root word is mercy, which comes from the French merci, "reward, gift, or kindness."
adj having or showing no mercy
having or covered with or accompanied by blood
used of persons or behavior; showing no clemency or mercy
lacking compassion or feeling for others
bowelless, cutthroat, fierce
ruthless in competition
unrelenting and deadly
pitiless, remorseless, ruthless, unpitying
without mercy or pity
resembling a tiger in fierceness and lack of mercy
incapable of being placated
showing or giving mercy
(used of persons or behavior) inclined to show mercy
showing or having compassion
compassionate and kind; conciliatory
marked or motivated by concern with the alleviation of suffering
having or showing a tender and considerate and helpful nature; used especially of persons and their behavior
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