This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Coronation Street
Anne Malone's death in 1998 when she was accidentally locked in the cold storage freezer in Fresco's while trying to set up Alma and Spider by tampering with the food products, after previously setting up Curly and then trying to shift the blame to Alma after Anne's falsely told he actually loves her. Her constantly calling for the security guard and banging on the door as she slowly froze to death was particularly disturbing.
As awesome and memorable as Richard Hillman was, you have to admit the circumstances around the murder of Maxine Peacock in 2003 was horrifying. She came home to check in on her infant son for a moment, she finds Emily (who was babysitting the boy) slumped over and bleeding profusely from a Tap on the Head, she sees Richard holding a crowbar in his hand, asks him what he was doing, to which he replies "You should have stayed at the party, Maxine." She then hears her son crying, she cries out and tries to run up to him, but he tackles her and proceeds to beat her to death in her own living room with her baby upstairs and while her husband and her mother at the latter's 50th birthday party none the wiser. And yes, she was left Dies Wide Open.
The 2017 storyline concerning Bethany Platt being groomed by Nathan Curtis. He probably could count as one of the darker main villains in the history of the show (which is saying a lot when you think of the numerous killers through the years). However Nathan has no redeemable qualities (concerning the type of plot he is in) and it is uncomfortable watching him slowly manipulate Bethany in order to pimp her out to his mates. And in recent episodes, it turns out he plots to sell her abroad. Worst of all, he has done this to numerous girls in the past and gotten away with it so far. He even seems to take pleasure in what he is doing, with a scene even having him delighted at hearing about parents doing a public appeal for their missing daughter (who was likely kidnapped by his group). The likes of Alan Bradley, Richard Hillman, Tony Gordon, Karl Munro and Rob Donovan had serious issues (with some of them even killing), but at least had some sort of humanity and some of these men showing remorse for their actions.
To make matters worse, one of Nathan's associates, Neil, is a police officer.