A cat hasn't the capacity to make moral and ethical decisions in the same way that most humans do.
It has animal instincts and reflexes as humans do but it has a very different sense of society (if any) it has a very different sense of self and empathy (if any).
There can not be an evil cat but a cat can do things that humans find loathsome enough to be classed as evil (like pulling a beloved pet to pieces or injuring a child or an old person).
All humans at sometime for some reason will have a distorted sense of society, self or empathy compared to the norm.
This increases the probability of committing acts of evil but it doesn't create certainties.
Sometimes people have a permanent distortion of one or many of those factors.
Being sexually attracted to children isn't evil, it's dangerous because it makes criminal acts more likely but there are good people with these impulses who go out of their way to seek help to control them at risk to themselves, not enough sadly but such people do exist.
Sex between children and adults is considered evil largely because of subjective cultural taboos (different societies have and have had different ideas about what is acceptable at different times, the goal posts of evil are moveable).
Generally a child lacks the intellectual and emotional maturity to give consent.
It is therefore a form of rape even if the child claims to be consenting.
Humans become mature enough to give consent at different times but in order to protect the broadest spectrum of people law makers have to set arbitrary ages of consent which should only be moved after research and careful legislation.
Some people who abuse children aren't sexually attracted to them, they are attracted to thrill of breaking a taboo or inflicting cruelty.
Some people who molest children are opportunists who see an avenue for sexual release which is easier to obtain and control than a preferred adult.
In some of these cases laws and safeguards are going to be more effective as preventative measures than others.
A sexual sadist is more likely to be turned on by breaking laws and navigating around obstacles than a sexual opportunist, who is more likely to seek easier gratification elsewhere.
Ultimately containment is the best solution.
If such people are identified they should be kept away from children for a long as possible.