Alec is definitely the antagonist of the story. He is a young, selfish richboy who won't take no for an answer. He stalks Tess and takes advantage of her when she is 16/17 years old and she is never the same after that. Granted, she never tells Alec about the baby until years later, but she didn't want the devil in her life forever, so she took all responsibilities for her life upon herself in order to keep him out. Alec might also seem like a dynamic character who repents of his old, youthful ways and converts to his religion. He even sets out preaching later on. But he is cursed by his desire for Tess and reverts back to his lustful, selfish, and evil ways after seeing her again years later. This proves that he is wishy-washy and unstable, but he never relents and he takes advantage of Tess's poor family circumstances once again when he provides for her ailing family. Alec represents bad habits that may leave, but they always come back to haunt us. From a woman's perspective, there's no wonder why Tess kills him in the end because that is the only way to get him out of her life for good.