i think there is a notable difference in the treatment of the dark/evil clary versus "morally gray"/evil/dark magnus tropes and people act like they just “like morally gray characters” and “dark stories/ships” but it’s quite transparent when you actually read how different characters are treated under this same trope/idea
like clary is treated as either Hot And Part of the Sexy Edgy Dark Murder Incest Trio so shes cool and murder is good actually and there's no abusive relationship here they're just all in love and a great murder ot3 or whatever... or like shes misguided and needs to be saved bc she went full dark no stars but its ok i promise shes a good person and she needs no redemption for her actions
so like, it’s treated as a good thing, either as in murder is totally excusable and/or not the point (like reading a story about a supervillain, sympathetic or not, often comes with murder. people still like harley quinn and sympathize with her despite the fact she has killed before and will kill again, and this is not a problem nor is it treated as such, because the point is her morals are fucky. the story isn’t saying she’s necessarily a good person, but also you gloss over her actions because that’s not the point.) so her actions are glossed over and fine, or she’s completely forgiven/excused because She Was Manipulated Into It.
while evil magnus, while the "evil" IS treated as hot, is a) deeply opposed in canon and ooc, and like directly opposes his characterization and the point of his story (and literally his worst nightmare and something he has always taken great pains to never become!), while clary's evil thing was literally in canon and she actually did, and b) more rooted in his treatment of alec, a white character, and him being toxic/abusive/cruel and "owning" alec. the hot comes from alec, poor uwu white boy, being hurt and abused by magnus, a man of color.
so magnus being evil isn’t a good thing, it’s almost always related specifically to him hurting alec (or someone else, but usually alec), and even if it’s not that, it’s about reveling in what a horrible monster he is, usually going right back into him being a downworlder (the subtext also connecting to him being a queer man of color, but more directly in the text, specifically his “demonic” nature) and it just being “in his nature” which is an incredibly racist and terrible narrative even if we are talking about literal demon blood the idea you can have an inherently evil nature is in of itself fucked up
his actions aren’t sympathetic (despite the fact that even if we did throw out his characterization entirely and assume he became evil under asmodeous’s tutelage, he was literally like, an abused homeless child being manipulated by a much stronger and older man, who was his father and also a literal demon, so if anything he would be far more sympathetic than clary who just woke up one day and decided to murder) and he and his actions are directly condemned in a way clary isn’t. he’s just. a bad person. his actions are shown as bad things that he did. he is a horrible person.
evil clary does something bad--hot. glossed over. not the point. or just forgiven and waved away. she’s hot, sexy, and effortlessly murdery. excellent.
evil magnus--look how terrible he is! what a monster, doing something like that to such innocents! disgusting! he’s evil and we think it’s hot to make him a complete monster! look at him hurt alec! how horrible!
not to mention again i can’t emphasize enough no matter how dumb it was clary canonically went evil full dark no stars and decided she wanted to murder for little to no reason... even tho she’s been through shit (nowhere near as much as magnus, and she’s still a privileged cis white shadowhunter, not to mention in canon, straight, despite the fact one can make good arguments for her being queer) but like she did go evil. that happened. so evil/dark clary, while often mishandled, isn’t necessarily against her character inherently. and it’s not a core part of her character to like, not be Like That, unless you buy into the angel blood = good nonsense, in which case, i hate to break it to you, but that is as much as an (albeit badly handled) allegory for racism/white supremacy as “mudbloods” are in harry potter. so like. maybe don’t buy into that, even if the narrative sometimes does (and cassandra clare always does, not that this is about the books, but just saying, she bought into her own fantasy racism because she’s that fucking bad)
magnus tho? dude. duuuuuuuuuuuude. the whole fucking point is that he’s been through endless shit, countless horrible things, deep trauma, and like. he’s still not fucking evil or dark. this man has been living as a gnc bisexual downworlder of color for like four centuries (whether you believe he’s trans or not he still falls into enough Minority Categories(TM) im pretty sure he’s got the “has been discriminated against and treated like shit a lot” thing covered), and he has gone through abusive relationships (romantic and familial) and lost friends and lovers, he’s been through horrible times where his people were hunted down like animals, he’s been personally hurt and abused over and over again, he’s been lonely and terrified and hurt and angry and scarred... and yet he always, always chooses kindness. he stays soft and vulnerable and kind, he stays selfless and full of love despite it all, which takes real fucking strength. he keeps caring, he doesn’t go numb, he doesn’t get cruel, he doesn’t ever go dark like his father wanted him to, like shadowhunters told him his blood made him. he isn’t dark, and that is the fucking point. that is what makes him inspiring, that’s why he’s compelling, and it’s like a huge fucking part of the core of his character. the whole goddamn point is he helps people and he’s kind and caring despite all the reasons he’s been given not to be! he made his choice a long time ago when asmodeous tried to make him dark, and he will never go back. that is literally his worst nightmare, something he still fears in himself to this day despite literally being the furthest thing from being anything like his father! because he’s a good fucking person! and y’all just want him to be evil so you can wank to your racist fantasies of him abusing your white fave.
but sure, you just like dark, edgy, morally gray things. there is definitely nothing more to that. sure.