Only an idiot would be happy that an animal attacked people.
When we had our daughter, my cat thought she could attack the baby. First time my daughter got clawed, the cat got herself a one way ticket to the shelter. I was not going to put an infant, especially my daughter, at risk of being hurt, just wasn't going to happen.
I like animals, including cats, but attacking people is unacceptable behavior in an animal. No one was ever mean to or hurt my cat, she just decided she didn't want to share her house with our baby. Or maybe she was establishing dominance, who knows... attacking a person is not permitted though.
Saying you are happy that a cat went bonkers and probably had to be killed is just sick.