"Once victim, always victim—that's the law!": What happened before and after Tess says this to the character of Alec?
Before Tess cries out to Alec Stokes-d'Urberville "Once victim, always victim—that's the law!" she has been abandoned by Angel (who is not such an Angel: "[Mr Clare] loved his misnamed Angel") and has extended her earlier self-imposed exile by taking work in the unforgiving land and climate of Flintcomb-Ash farm. On her way there, she happens upon an itinerant preacher who turns out to be a reformed and repentant Alec. He pursues her. They walk and talk with nothing but rejection and discouragement toward him from Tess. Alec is overpowered by the desires Tess reawakens in him and renounces his religious conversion, then pursues her to Flintcomb-Ash. Here, he relentless forces his notice upon her, continually trying to persuade her to reunite with him despite his knowledge of her estranged union with Angel.
It is during one of their conversations that Alec says to an utterly exhausted and hungry Tess that she has caused his spiritual "backsliding" and that she is married to a "mule" of a husband. After all else Alec has subjected her to, this is too much for Tess. With her heavy leather work glove, she strikes Alec across the mouth. He and she both spring to their feet in rage though both exert further self-control thus ending the scene except for the words that are to come.
Tess cries out "Now punish me!" saying that she will not cry out for help because once someone has become a victim of punishment, they will always remain a victim of punishment according to a universal law: "Once victim, always victim—that's the law!" Alec ends the scene by threateningly reminding her that he had been her "master" once and would be her master yet again.
One of her leather gloves ... lay in her lap, ... she passionately swung the glove by the gauntlet directly in [Alec's] face. ... Alec fiercely started up .... A scarlet oozing appeared where her blow had alighted,....
She too had sprung up, but she sank down again. "Now, punish me!" she said, [with a] sparrow's gaze before its captor twists its neck. "Whip me, crush me; ... I shall not cry out. Once victim, always victim—that's the law!"
"O no, no, Tess," he said blandly. "I can make full allowance for this.....
"But remember one thing!" His voice hardened .... I was your master once! I will be your master again."
Alec leaves for the moment. When he returns late that night, he persists in trying to gain Tess's affection and agreement to leave and go with him while promising to care for her and family:
[Alec] "I can make them all comfortable if you will only show confidence in me."
Tess is prompted to write and actually send a letter to Angel in care of his parents' home in which she pleads with him to accept her back even if only to live with him as a servant and not a wife:
"I would be content, ay, glad, to live with you as your servant, if I may not as your wife; so that I could only be near you, and get glimpses of you, and think of you as mine."
Her impassioned plea was forwarded to Angel in Brazil by his parents on the eve of his preparations to depart and return to "fetch" Tess. Had they both communicated more freely and sooner, much might have been averted, especially after she cries out to Alec as a victim.