The Darkness Within is Episode 3-3 of Trauma Center: Under the Knife and Second Opinion. It's a story-only episode.
Story[]
While looking for Tyler, Derek finds and reads through some documents on a table. The documents comprise a thesis on euthanasia, alongside a letter thanking Tyler for euthanizing the writer's husband. Tyler comes in, and realizes that Derek has discovered his background. Rather than try to defend his stance rightaway, he decides to lead Derek to his sister, Amy Chase, who's been infected with GUILT and hospitalized at Caduceus.
After being introduced to Amy and her nurse Leslie Sears, Tyler admits that he's been practicing euthanasia in secret all this time, after watching one of his professors suffer from cancer and die an agonizing death. He feels that euthanasia is justifiable if the alternative is living a painful life while delaying the inevitable, and implies that he's been contemplating the same fate for Amy if her GUILT infection proves to be incurable. Derek is staunchly against this.