From 3x10, Lucrezia having to Mercy Kill Alfonso with poison, so he doesn't have to spend the last few days of his life in agony after Alfonso's foolish attempt to duel Cesare. Afterwards, all she can do is lay next to his body in a catatonic state, covered in his blood from her earlier attempts to save his life.
Alfonso: If you ever loved me, you will do me this one last favor.
Lucrezia: I cannot.
Alfonso: Yes, you can. You're a Borgia.
- How about the fact that she was the one who found him on the floor? With Cesare standing over him and holding his bloody sword?
- Or how she asks a silent, guilt-ridden Cesare if all she can ever be in her life is a Borgia? As she leaves the room with the poison, Cesare can only mutter 'A professional,' echoing the words he's spoken to Rufio just earlier. Cesare sits there with the realization that his actions have completely destroyed whatever innocence or joy that his beloved sister had left in her.
- An earlier one occurs when Lucrezia confronts Rodrigo in a confessional booth, tells him that she knows about his and Cesare's designs on Naples and begging him to tell her if their plans call for the 'removal' of her husband. Worse is when she bitterly states that his ambition, now in unity with Cesare's, has taken her place in his heart, and then cutting him off when he tries to reassure her that's not the case. Compare Lucrezia's relationship with Rodrigo from the beginning of the series to where it is now, and the difference is clear.
- The worst is that Rodrigo can neither lie to her nor tell her the truth. She takes his silence as confirmation of her hears for her husband and leaves.
- And what about poor Alfonso? At his core, he was nothing but a well-meaning, sweet boy who had fallen head over heels in love with Lucrezia. And from then on, he's involved in so much intrigue and machinations that he can't see straight. His own uncle is murdered, his cousin holds them both hostage, and just as they escape, he's forced to confront the rumours about his own wife (that he loves utterly) and her brother. He starts drinking, becomes increasingly paranoid and when Cesare confronts him in his home, he already knows that he's going to end up just like Juan. Just listen to him as he brokenly tells Lucrezia that Cesare has killed him...'just like you both wanted.'