Lucy knows guilt isn’t what is going to make her powerful. And she can’t let herself have any distractions while being on a mission, something, or the only thing she holds to dear life.
Even if it might be cruel in someone’s life or something, according to her, her ambition is more enormous and is helpful for the greater good so she wouldn’t mind stomping on someone’s life if that’s what it takes. It just doesn’t have to be hers after what she went through.
She knew it was hard for weak people to survive, so she killed all the weak parts in her to stay, thriving to win. And guilt is one of them and she definitely wouldn’t let it pull all her hard work down.
She seduced Alexa’s lover even though she knew that shouldn’t be done. That wasn’t in her plan after what Alexa did to her, but she, fortunately, found out by throwing stones in the dark that she could actually seduce a man with the magic power she has in her.