Alec is introduced as a twenty-four-year-old player with a stage villain's curled mustache. You know he's going to be the bad guy from the start, but Hardy is careful to make him seem human, too. He has ruined Tess's life, and is totally despicable, but he still develops as a character. He converts to Christianity and actually begins traveling around the county, preaching in fields and at fairs to anyone who will listen. He gives it all up when he sees Tess again (unsurprisingly), but these dramatic shifts in his character help to make him seem like a more realistic, human character, as opposed to a simple, one-dimensional villain.
Tess's troubles begin with Alec, it's true, but the guilt and shame she feels after being raped is a result of the conventional morality imposed on her by the outside world. Victorian society made pre-marital sex into a crime that was solely and entirely the fault of the woman involved. Those conventions are what drive Angel to reject her after she's confessed to him. The hard-heartedness of the world in general makes Tess vulnerable to Alec's persuasion after Angel has left.