All in the Family had the serial rapist who attempts to make Edith his latest victim on her 50th birthday.
To a lesser extent, the kids who attack Mike and Beverly LaSalle also count. Though neither the kids, nor the incident itself are shown, their act of beating Beverly to death causes Edith to (temporarily) renounce her faith in God in one of the series's darkest episodes.
Breaking Bad, though quite a macabre show from the start, retained a prominent Black Comedy element in its first and second seasons. This was chipped away in stages by a series of particularly menacing and/or cruel villains.
Furthermore, Todd Alquist extinguishes any light-heartedness left in the series with his on-screen murder of a child, an event which creates a permanent rift between Walter and Jesse, who had just been working alongside him.
Lastly, his uncle Jack Welker is much more monstrous; unlike Todd, he doesn't brutally kill out of ignorance of understanding, but for sick pleasure. He then kills Hank and though he entertained Walt's pleas not to beforehand, he never intended to let Hank live.
And when Lord Zedd started losing his edge we got the superficially goofy Master Vile... who had much stronger monsters, which would routinely trash the rangers, and force them into a corner while he came closer than ever to ruling the earth. Even his monsters of the week were far more dangerous than Zedd's, having annoying special powers and almost never being defeated in a lame One-Hit Kill zord battle. And he's easily the most Genre Savvy villain that Power Rangers has ever had; after many failures at defeating the Rangers, what does he do? He goes back home, where as he puts it, the bad guys always win.
Before Trakeena or Deviot, there was the Magna Defender. Not a villain, strictly, but just as bad at first - the series' first Anti-Hero, he was on a Roaring Rampage of Revenge and willing to sacrifice innocents (albeit reluctantly) to do it.
Divatox is a weird example. In the movie, she was supremely goofy and whiny, played 100% for laughs, and the evil demon-thing Maligore served this role. In the TV series, at least for the first half of the season, she got a new actress and was a bit more serious. Ironically, when the old actress returned and went right back to being ridiculous, she destroyed the Rangers' base and powers successfully, leading to the season's Cerebus Ending. Viva la Diva!
It would probably be more accurate to apply this trope to Goldgoyle. At first he looks like just a Monster of the Week, but then he No Sells the Rescue Megazord's Finishing Move and destroys it and the Turbo Megazord, with the Rangers only stopping him by sacrificing their weapons. The damage he deals is another sign that things aren't going to end well.
Power Rangers Samurai has Serrator, who appears near the beginning of Super Samurai. Not only is he responsible for making the Deal with the Devil with Dayu that turned her into Nighlock, in the process erasing Deker's memory and turning him into a half Nighlok cursed to walk the earth searching for the ultimate battle (which wasn't part of the deal, Dayu thought she was saving his life!), which he admits to Dayu's face that he enjoyed every minute of, he's leagues beyond the Rangers and routinely beats them down. On top of this, the Nighlok he brings with him are pretty much living torture devices.
Not to mention why he did all these things. He needed Deker the way he eventually became, and the suffering his Nighloks caused as raw material; it's all part of the Long Game to do in one stroke what Xandred and company were trying to do one monster at a time and he nearly succeeded. He didn't do what he did For the Evulz; he's quite a Magnificent Bastard. And even once he was gone, the consequences of what he did to Deker and Dayu only got darker. In the end, since Dayu's grief over Deker's Final Death is what allowed the injured Xandred to restore himself and become immune to the sealing symbol, Xandred going from Orcus on His Throne to the real menace he was in the finale is also courtesy of Serrator.
Power Rangers Dino Thunder had Mesogog. In comparison to just about every other Power Rangers series, nothing about Mesogog was played for laughs, and he never cracks jokes or engages in comedy antics. He also looks pretty damn menacing, and his voice was a perfect complement.
Power Rangers in Space, while it started off more serious than the its predecessors, it really started to move into Dark And Edgier territory with the introduction of Darkonda. He shows himself to be far more ruthless than Astronema or Ecliptor, and he's revealed to have also kidnapped Astronema as a child and caused her to raised to be evil. He later helps bring about Ecliptor being brainwashed to make him more evil, which leads to the same happening to Astronema, after which the series starts even more serious.
Power Rangers S.P.D. has this with Emperor Grumm. A menacing living skeleton who is established to have taken over and/or destroyed many, many worlds before the storyline even began. He has a handful of less-than-menacing moments with Mora, but that's it.
M*A*S*H inverts this. Its one-shot villains, usually a Colonel or General, often have a high casualty record or disregard for human life. The recurring 'villains' like Burns or Flagg are comically inept.
In Kamen Rider Ryuki, Takashi Asakura/Kamen Rider Ouja was the embodiment of this trope (formerly he was a serial killer). The Rider War consisted of thirteen riders fighting each orher to the death for a single wish that the real big bad planned to exploit for himself and it wasn't exactly puppies and sunshine, but before Asakura came on the scene, only one Rider - a clearly evil one, at that - had died, and the current 5 at that time weren't doing all that good a job at the whole killing each other thing. That changed once he became Ouja, and he's responsible for the bulk of the dead Riders. His first kill was Kamen Rider Gai/Jun Shibaura who was an Asshole Victim at best. The worst part came where on an attempt to kill Shinji/Ryuki, he killed off Kamen Rider Raia/Miyuki Tezuka, who along with Shinji were the nicest and actual heroic Rider OF THE SHOW. His third victim was Kamen Rider Imperer/Mitsuru Sano, who was tricked by Kamen Rider Tiger to attack Shinji, who (Tiger) literally stabbed him in the back, leaving him barely alive. Asakura found him (Imperer) and killed him just because his (Imperer's) Mirror Monster interrupted his revenge match against Tiger. (The worst part about this is that Sano had one of the worst tragic backstories in Kamen Rider history, and Ouja killed for just interrupting a fight. His last victim was his rival Kamen Rider Zolda/Kitoka who was an Anti-Hero at best though it was revealed that Kitoka died due to his cancer, so his butler fought in his place.
Buffy the Vampire Slayer: Angelus. Not that the show was exactly family friendly up to that point, and plenty of the villains had been genuinely dangerous, but from mid season two (when Angelus turned up) onwards, the show got noticeably darker. Some mention has to go to Spike as well, as he was the one who kicked off the storyline.
This trope is somewhat inverted with the Nerd Trio in Season 6. Season 6 is the darkest season as many bad things happen to the protagonists, but when the Trio are around, despite being the villains of the season they seem to lighten the story up with their comic relief and pop culture references. Although Warren soon proves to be just as evil as the previous Big Bads of the series, if not more.
For the Spin-OffAngel, Daniel Holtz qualifies big time. While Angel was pretty dark anyway, Holtz was a sign that Angel's quest for redemption may be doomed to fail. He manages to put resident Magnificent Bastard firm Wolfram and Hart under serious pressure when he manipulates them, successfully causes a long-term fracture in the team, and despite dying he basically achieves his goal of revenge, in fact his death is all part of his plan, which works and has consequences which last long into the last season.
The iCarly episode iPsycho, we're inroduced to Nora, an Ax-CrazyFangirl who trapped the gang in her basement to have them stay at her brithday party. Then we were introduced to her family in iStillPsycho and we saw the most terrifying, adventurous situation Dan ever put the gang in!
And that spilled over for her appearance in the spin-off Sam & Cat, which upped her actions to Buffalo Bob levels.
Scorpius in Farscape. Within a few episodes, he tortures Crichton and usurps the role of Big Bad from Crais.
Serial killers Mr. Yin and Mr. Yang on Psych. The latter played mind games with Shawn, and later hooked Shawn's mother up to a bomb; and the former murdered Mary, kidnapped Juliet and Abigail, placed them into death traps to taunt Shawn, and got away scot free (the first time). A terrified Shawn even remarked during the Yang case that his constant wisecracks were a coping mechanism to keep himself sane. But since he had to focus all his energies on solving the case, he asked Gus to pick up the slack in the goofiness to keep him from cracking under the stress (with hilarious results and lots of awkward looks from everyone else).
Supernatural: when Mary Winchester showed up in Season 1's "Home", everything started going downhill. The writers even lampshade it in the commentary. After they realized how well Jensen and Jared worked together, they made everything more emotional, darker and less comedic...if you ignore "Yellow Fever", "Hollywood Babylon", "A Very Supernatural Christmas", "Mystery Spot", "Monster Movie", "Fallen Idol", "Changing Channels", "The Curious Case of Dean Winchester", "The Real Ghostbusters", " Sam Interrupted" and "Bad Day at Black Rock".
Hell, considering the latest villains in Naomi and Enoch and what's happened in season 8, fans actually miss Lucifer. As in, the devil himself. It's come to the point that the more light-hearted episodes are probably the only thing keeping the fandom sane, and considering how even those have moments of pain and angst in them, that's not saying much.
Ashes to Ashes has two fairly spooky but ultimately ineffective villains in season one's Clown (who's more a mental/emotional villain for Alex than a true threat) and season two's Martin Summers, who doesn't directly hurt anyone. That changes in season three, when Jim Keats is brought in.
It's pretty hard to pin down the exact point when Castle has started to become darker, but the 3XK certainly fits this trope. Especially since his introductory episode was the series' first one to have a real Downer Ending.
Also a case of Crouching Moron, Hidden Badass. At first he seemed comedic, with the doll on his arm and messing with it being his Berserk Button, and it would always move while offscreen. While a Mad Scientist, he was under Kougami's thumb, and was the only one who thought his working with Kazari was actually a secret. Nobody could have forseen that Creepy Doll Guy was going to turn the tables on everyone and become a dark and deadly villain and shoot past Kazari and Uva to the top of the heap.
Star Trek: The Next Generation initially gave us the Ferengi, an Ineffectual Sympathetic Villain race that was meant to be the new Big Bad on the level of the Klingons and Romulans from the previous series. This didn't quite work out, due to them being comically short trolls with a lust for money and rampant misogyny. Then the Borg were introduced, and became the number one ultimate threat to the Federation for the entire series, despite only appearing in six episodes. In their first encounter with the Borg, the Enterprise was utterly defeated and on the verge of being dissected and assimilated before Q rescued themnote After having exposed them to the Borg in the first place in order to give them a kick in their complacency..
Q: You judge yourselves against the pitiful adversaries you've encountered so far. The Klingons, the Romulans; they're nothing compared to what's waiting.
After two first season episodes as a zany foil for the Enterprise crew, Q himself becomes a much darker character by introducing the Enterprise to the Borg. When Riker calls him out for causing the deaths of the Enterprise crew, Q responds with "Oh please". When Picard begs Q to tell him that it's all just a silly illusion, Q says "Oh no, this is as real as your so-called life gets".
Dave Karofsky from Glee. The show's antagonist is usually Sue Sylvester and her Refuge in Audacity antics, but Dave, in addition to tormenting the glee club with his buddy Azimio, had a special hatred for Kurt. At first it just seemed like homophobic bullying, until Dave forcibly kissed Kurt. From there, his gayngst became a central plot point, going as far as threatening to kill Kurt if he said anything, forcing Kurt to temporarily transfer schools. Even after Dave's Heel-Face Turn, he himself transferred schools when rumors started flying that he might be gay, and it all goes to hell when a boy at his new school sees him professing his love for Kurt at a party and outs him at school, causing himto be ruthlessly bullied until he attempts suicide.
Justified was a dark morality play from the beginning, but it's Bo Crowder's release from prison partway through Season 1 that would ultimately see the show lose much of its comedic edge. Bo's actions turned Johnny into a crippled, bitter, scheming snake, derailed Boyd's quest for redemption, forced Raylan into an alliance with Boyd, made Raylan's relationship with his father, Arlo, even worse, and set up most of the darker elements that continue to effect the show into the present.
The Walking Dead: Glenn, Hershel and Rick meet Dave and Tony in season 2, two survivors who are prepared to kill them for their resources, introducing the main character to the danger of rival groups. These two random men quickly prove that normal people are an even bigger threat than walkers.
In the Season 5 episode of The Fresh Prince of Bel-Air entitled "Bullets Over Bel-Air," a nameless armed thief held Carlton and Will at gunpoint while they were at an ATM machine. He shot Will after he took the bullet for Carlton and runs away. Because of his actions, it lead to Carlton buying himself a gun as an attempt to protect himself and his family members and to the biggest Tear Jerker in the show in which Will begged Carlton to give him the gun when he was visiting him in the hospital.
The show Las Vegas was fairly light in tone despite several dramatic moments such as Danny's shellshock and the episode's typical villains were usually high-profile thieves, cheaters and con artists. Then comes Vince Petersen in season 4, who quickly establishes himself as the most twisted and horrifying one when he is revealed as a rapist serial killer when he abducts Sam Marquez to do the same to her. After Sam kills him she suffers from PTSD in the next season and spends most of it trying to cope with the trauma.