I witnessed a cat attact a person for absolutely no reason. The person
was just standing there, minding her own business and the cat came up
to her and started biting and scratching her in attack mode. This was
a vicious attack. There was absolutely no provocation, I witnessed the
person's behavior from the time the person walked into the house.

I also know that the cat had never seen the person before so it was
not a case of having a grudge. Also, this cat had never been
mistreated, as far as anyone knows. It had been the family pet for
many years. No one knows what caused the attack, but it was totally
unprovoked.

There have been other occations where I have seen a cat bite and/or
scratch a person who was playing with them or brushing them, or so
forth. This is almost always done with no anger, it is a case where
the cat is overstimulated and the agression is displaced towards
whoever is close. That can be a problem for inexperienced people, but
I do not consider these cats as bad.

I have been bitten by cats, again displaced agression, but I do not
consider those particular cats bad, it was simply the situation, they
were frightened. I have been bitten and scratched by cats that did it
just because they were anti-social and happened to be within striking
distance. SOME of those could be considered bad cats, others just need
some socialization.

Others tear up furniture, intentionally knock over things on shelves,
and attack other animals. Some of those are considered bad, but
sometimes they are simply bored.

Sometimes people consider a cat that does not use the litterbox as
bad. That is almost never the case. It is almost always a situation
that can be rectified with a little work on the part of the owner in
discovering the cause of the problem.

To paraphrase another reply to this question I quote (loosly), the
late Father Flanagan, founder of Boys Town "I have never met a bad
boy." My response to that is that the good father should meet some of
the boys that I have met. LOL.