Alec is deceitful, manipulative and a bully, and, like all bullies, he is easily manipulated himself by another bully or officious person. This is why Angel's father was able to coerce Alec into a radical change of lfe and also why the change could not hold up when presented with external facts or attractive alternatives.
"I have arranged to preach, and I shall not be there—by reason of my burning desire to see a woman whom I once despised!—No, by my word and truth, I never despised you; if I had I should not love you now!"
D'Urberville is deceitful and manipulative from the first, and it is these traits of his that lead Tess to her doom--she has to be tricked by vile means to succumb to Alec and his traits make him an expert at trickery. It is through trickery that he also begins to approach Tess in the hope that he will win her over to reuniting with him:
D'Urberville in anger retreated from her to the hedge, where he pulled off the long smockfrock which had disguised him; and rolling it...
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