Do you think it's the TV show writers who ruined Allison's character? Well, if you look closely at the comics, the issue becomes debatable at best.
In 'The Umbrella Academy' season 3, Allison does enjoy some character development – if one can call it development, of course.
Clearly unable to make peace with the fact that she continues to lose everyone she loves, Allison initially starts to snap at the siblings, and then things escalate to a point when Number Three exercises her rumoring power to make Luther "want" her, only to finalize her character journey with the murder of Harlan – an autistic boy who received supernatural powers after Viktor saved his life in the 60s.
Was it Allison that we knew from the comics? Well, it turns out that… yes.
With fans resurfacing comic arcs of Number Three, they found that the comic version of Reginald Hargreeves described Allison as "an insufferable, narcissistic creature" who "prevaricates with appalling ease". In fact, the scene of Allison sexually harassing Luther via rumoring is even depicted in the comics, even though the circumstances are somewhat different and she persuades him that he "wanted to kiss her since he was eight".
The realization is quite painful to 'The Umbrella Academy' fans who never read the comics, but those who did now observe the "Allison hate train" with a certain degree of condescendence, given that they knew that Number Three had a controversial character arc from the very beginning.
"I know basically everyone hates Allison for what she did... But it's following the comics, we knew this was going to happen... It wasn't right but it wasn't surprising," – @09StephE.
Still, some people think that the show's writing is also to blame for how hated Allison's character has become.
"I think they're trying to make her more like her comic character who's more like a villain but the things Allison did in this season were so??? Like we could've made her more accurate to the comics with better writing and without some things she did like what she did to Luther," – @jairiasss.
A certain group of fans, while acknowledging that Allison has become "unbearable", applaud the character integrity.
"Anyways contrary to popular belief i for one think they rly made Allison's character as awful as she is in the comics somehow. Good for the character integrity but she was literally unbearable," – @wanyueism.
So yes, it looks like Allison has been the villain all along, even though the first two seasons of 'The Umbrella Academy' have established her to be quite an empathetic and likable person, albeit chaotic and controversial to the extent that is attributable to pretty much everyone in the dysfunctional superhero family that the Umbrella Academy is.