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."