15. Nygma Kills Kristen Kringle (Season 2, Episode 6 - 'By Fire')
Things didn't go to plan for Ed Nygma (Cory Michael Smith) when he made a much-awaited confession to Kristen Kringle (Chelsea Spack) back in Season 2. The situation soon escalated after Kristen panicked upon finding out that Nygma had killed her abusive boyfriend, Tom Dougherty - even faking his disappearance.
Before this point, Nygma had displayed some of his Riddler-like qualities but he hadn't truly turned full supervillain just yet. That was all about to change. When Kristen wasn't grateful for what Ed had done, he killed her too - right then, right there.
Clearly this murder was a lot more difficult for Nygma to commit - after all, Nygma loved Kringle. He truly believed he'd done her a favour, but once she was gone, so was the last positive moral influence on his life.
Nygma was a popular character in Gotham throughout its five season run, but there is no doubting that the most thrilling element of his arc was his devolution into becoming the Riddler. Nowhere was that clearer than here.