Why does animal cruelty seem somehow worse than human cruelty?

Animals are innocent. They have no concept of violence for the sake of violence. In that sense they are purer than us humans, and you are right to feel pain at cruelty towards purity. Such aggression is our making, and it is best kept to ourselves, which is why it hurts less when such cruelty is used on a human. A human is capable of understanding such things. A human can use that knowledge to cope with it. An animal cannot.