Short answers, pulled from the story.
An antagonist is a character or force in a story presented as the main enemy or rival of the protagonist. The role is used as a plot device to create conflict, obstacles, and challenges. The antagonist does not have to be evil or even a person; natural forces and societal norms can also fill the role.
No. In Death Note, the protagonist Light Yagami is the villain and the antagonist L is a hero working to stop him. In Macbeth, Macduff is the antagonist yet is arguably morally correct in opposing the tyrant Macbeth. The antagonist is defined by opposition to the protagonist, not by moral alignment.
A villain is a character who makes morally objectionable choices, while an antagonist is any character or force that opposes the protagonist. Javert in Victor Hugo's Les Misérables is a classic antagonist with no malicious intent; he represents the rigid application of the law rather than personal evil.
Yes. A tidal wave that destroys a city, a storm that causes havoc, or the conditions of a particular environment can all serve as antagonists. Societal norms and rules may also occupy the antagonist's role, pressing against the protagonist without any individual directing them.
J.R. Ewing from Dallas, played by Larry Hagman, and Alexis Colby from Dynasty, played by Joan Collins, are notable examples. Both became breakout characters used as devices to increase their shows' ratings.
John Truby argued that a true opponent not only wants to prevent the hero from achieving their desire but is competing with the hero for the same goal. In his view, it is only by competing for the same goal that the hero and opponent are forced into direct conflict again and again throughout the story.