I mean, whether you're talking DW TOS or current DW, Doctor Who is rarely truly dark. When it does get dark, it usually does so in particular episodes, but then offsets that later on. E.g., "Vincent and the Doctor"'s ending is heartbreaking, but it's offset by "The Lodger" a week later. "The Waters of Mars" and "The End of Time, Parts One and Two" are dark, but they're offset by "The Eleventh Hour," "The Beast Below," and "Victory of the Daleks." Etc.
Really, Doctor Who by definition can only get so dark. It remains, after all, a family show with a core target audience of 8-year-olds. Even when it gets dark -- "Dalek," "Human Nature/The Family of Blood," etc. -- it has to find ways to moderate that darkness, to present it in a manner that's emotionally safe for its target audience to assimilate. You're never gonna see Doctor Who go the route of Breaking Bad or Six Feet Under or The Sopranos.