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In Beginnings, it's openly stated that L relishes the idea of putting Misa in restraints in case she loses her temper (compare this to his canon counterpart who treats tying up Misa as a necessary evil and shows no evidence of explicitly enjoying doing it) when Light tells her that he loves L and is leaving her. The fact that Light was the one that’d been leading her on up until this point is hardly if at all addressed and the story treats it as excusable because at the time Light was Kira and therefore "not himself" and besides who cares what Misa thinks anyway? She's crazy and as dumb as a post. When she threatens to commit suicide ("to give him a shock to snap him out of it"), L looks for a window to open so that he may “accommodate her” (again, compare to his canon counterpart who appears visibly shaken about the deaths of Ukita and the FBI agents and has Watari stop Misa from killing herself in confinement by gagging her so she can't bite her tongue). Later when Misa leaves, Light tells L that he knows that he "didn't mean it," but L admits that "the thought was enjoyable, anyway." To make things worse, the author seems to want the reader to side with L and Light and somehow this attempt at a “Take That, Scrappy! is supposed to be cute and funny.
Then there's the shocking plot twist that reveals that Light is actually Blud the Shinigami and Omnicidal Maniac who wants to kill everything and everyone simply because he can. Also L is responsible for the events that lead to the start of the series. His goal was apparently to create some sort of detective version of Outer Heaven, by creating a world where crime and Kiras would always exist, and thus, the world would always need detectives.
In My Stupid Reality L isn't satisfied in just collecting genius orphans in his search for a successor and with help from Watari begins kidnapping and disappearing genius children from around the globe and putting them through reckless testing and experimentation. Many are Driven to Suicide and those that return are like Empty Shells.
In The Shinigami Wing Deal Souichiro is a homophobic Jerk Ass, Mikami becomes a kidnapper pedo-rapist, Misa is very stupid, and L is more interested in getting into Light's pants than solving the Kira case.
Speaking of, Light and Misa try to kill Erin, an innocent girl who confronts them just as Light is in the process of writing his own father's name in what he thinks is Misa's notebook. It's assumed that Light might have gone on to kill everyone else afterwards had he not been thwarted. From their standpoint, it was probably justified, but still. Actually, it might be accurate to say that for L, Light and Misa, this trope along with Draco in Leather Pants are zig-zagged with each other.
In Those Who Stand for Nothing Fall for Anything Matt is a womanizing douchebag that makes Light'sJerkass tendencies look nice in comparison. Later in the story L physically abuses and rapes Light. Also in this 'verse he's the one using Ryuk's notebook to further Light's career by killing off threats and competition, even his own boyfriend Stephen.
In To Feel Alive Yamamoto is once again Light's abusive ex-boyfriend.
Here's another fic where Soichiro becomes an Abusive Parent: What Is Justice?, in which Souichiro regularly beat Light and Sayu throughout their childhood. It's also implied he had their mother killed. This fic makes another Ron The Death Eater with the surprise plot twist where sweet little Sayubecomes Kira.