i love lucrezia's inconsistencies about alfonso. she didn't want to get married in the first place, but the man that she chose is a gullible boy who reminds her of her own lost innocence. she says that she could love him because he's good, yet every single scene of them is lucrezia trying to change him (she tries to make him break his saint agnes vow and tries to make him more ambitious).
alfonso says that he loves her, but lucrezia doesn't believe it because alfonso doesn't demonstrate it the way cesare does. because no one can love her like cesare can. lucrezia is deeply in love with her brother, so no man will ever compare to cesare. but she understands that she can't marry him, so she tries to settle with alfonso.
she claims to love alfonso, but it is an almost maternal kind of love: she teaches him what she knows about power and survival, and he refuses it as a petulant son would. a love of the soul. not like brother and sister, of course. but still... alfonso is the most beneficial, logical choice of a husband. he doesn't hit lucrezia, doesn't try to control her, and although he is, frankly speaking, an asshole to her (the drinking, not defending her, "you're a borgia", etc) he isn't giovanni. lucrezia knows that she could do much worse, but she also knows that she could do so much better.
she wants to mold alfonso so she can get power. she doesn't want him to die, and she doesn't want to kill him. not because she loves him, but because it would be her fault. she poisons him out of mercy. alfonso is what she could have been if she wasn't a borgia, but she can't escape herself. in a way, alfonso is lucrezia's ursula: he thinks lucrezia is one thing, is deceived by it, finds out the truth, and pays the price. his death ends lucrezia's arc with corruption. her own borgia's stain.
