Maybe because rats have a bad reputation in real life. So, in cartoons, they are easily made into villains for that stereotype.
Maybe it's for adding a funny or unexpected twist. Making something ordinary like a trash can the bad guy creates surprise and entertainment.
It's used for comedic effect. The show often exaggerates physical traits for laughs.
Maybe it's for comical effect or to show a character's bad habits.
It could be a sneaky cat thief who steals food from other cats.
Sometimes, in some cartoons, Hercules can be shown in a negative light, depending on the story's plot and the creators' vision.
Well, sometimes it's to create contrast and drama. By making angels bad, it shakes up the usual good vs. evil dynamic and keeps readers or viewers engaged. Also, it might reflect society's skepticism about supposed paragons of goodness.
Bad kids in cartoons might be there to add drama or make the story more interesting. It also gives an opportunity to show how good behavior can overcome the negative influence of the bad ones.
Maybe it's because of his controversial policies and actions that some people find disagreeable.
Sometimes, in certain cartoons, Robin Hood can be portrayed with negative traits, but it's not a common depiction.
Maybe it's because alligators have that fierce look and sharp teeth, which makes them seem scary and suitable for the role of a villain in cartoons.