A good example of an antagonist would be Lord Voldemort, the notorious dark wizard in the Harry Potter novels of J.K. Rowling.
If you've ever followed or watched the Harry Potter series or read the Harry Potter Books then that would be an easy character to reference as an antagonist.
Another example of an antagonist would be the Queen in Snow White and the Seven Dwarfs, who opposes and wants to destroy Snow White.
For the most part being a protagonist is a good thing.
However some protagonist may be bad people but they may also do good things when they come up against an antagonist.
A protagonist is usually the hero and is good but the protagonist can sometimes be a bad guy in a story as well.
An antagonist is the bad one and is an evil person so if you're an antagonist then you're not the good person and you do bad things.
The protagonist would be the one against the antagonist.
An antagonist is always the bad guy in a story so you don't want to be an antagonist but it's good to be a protagonist.