The actress plays Debra Morgan on the Showtime drama. In the season finale, Debra let two killers go free, unbeknownst to her that one was her adoptive brother Dexter (Michael C Hall).
Carpenter told TV Line: "It went against everything I believed to be true about Debra and how she functions in her life and her work, where things are either right or wrong.
"The way I was able to get behind it was by going back and revisiting the first season. I really tried to draw out the feelings Deb had about being engaged to a serial killer. It was about redemption, taking things into her own hands, finding closure finally for what happened there."
Carpenter also called the final day of the season "incredibly emotional", partly due to the aftermath of the scene. However, she added that she is happy that her character ends up in a relationship with Quinn (Desmond Harrington).
"It was great," she continued. "Her love came in a totally unexpected package. I don't think Deb ever saw herself dating Quinn, but he's solid.
"He is exactly who he says he is and never deviates wildly outside of his character. I feel like Deb deserves that."
Dexter is to return for its sixth season in the fall. An executive producer recently teased that Dexter will be a "more evolved serial killer".
> Dexter: Julia Stiles chats season five
> Recap - Dexter: 'The Big One'
Watch a teaser trailer for Dexter season six below: