Cancer Cell:
Steroids:
I decided to propose those two.
What’s The Work?
Cells At Work is a Anime & Manga series about human versions of the Cells of the human body of a Red Blood Cell who always get lost doing deliveries and White Blood Cell always coming to her rescue.
Who Are They?
Cancer Cell?
Cancer is a major antagonist that is Cell mutated where he attempts to multiply and overrun the body in the first season and second half of season 2.
Steroids
Steroids is minor antagonist in Cedar Pollen Allergies where it is medicine sent to suppress immune response.
What Makes Them Heinous?
Heinous Standards?
This series kinda a weird case, while there is other villains the two can’t be compared to them due to the fact they’re hostile species who can’t be morally ranked and stand out from each other like Influenza, Virus, Rhino Virus, and others named after diseases to name the few while they are a individual villains thus he stands out by default. Even then all they are minor nuisances or don't do anything too harmful.
Heinous Actions?
Cancer Cell
He would start dividing making copies of himself where he would steal nutrients which end up hurting the body and he plans on doing it until the body dies where he attempts to kill White Blood Cell, Killer T-Cell and Natural Killer Cell too before meeting his end.
After his revival he attempted to seek revenge on White Blood Cell, Killer T-Cell (now Memory T-Cell) and Natural Killer Cell where he attempts to overrun the body again while managing to manipulate Regulatory T Cell to be his pawn for a time in attacking them before she realize he is a Cancer Cell where he attempt to kill them before meeting his end a second time.
In the Live Action film he nearly comes close to killing the body where his actions forced the body to get radiation treatment which wiped out nearly all the cells in doing so.
Steroids
It would attack the Cells causing destruction and mayhem in the suburbs while it tries to suppress immune responses until it’s battery runs out after a long time thus endangering the many Cells in the process here.
While it isn’t heinous as Cancer Cell, given it has a limited battery time and only appeared in one episode, endangering the Cells and causing destruction around suburbs is enough to stand out with less time than he did.
Possible Preventing Factors?
Cancer Cell
He has a clear moral agency and is aware his actions would kill the body but didn’t care at all
He doesn’t rely on Fridge-Horror either as if the body dies it is basically considered world ending meaning all the trillions of cells would die.
Steroids
He is the only Steroid that appears in the series and while more appear in Code Block it is unknown if Code Black is canon due to it being a different body but isn't confirmed if it is in the same universe.
What Makes Them Inconsistent?
Cancer Cell
He is too tragic for Near Pure Evil as he was hunted down as a child due to his mutated genes by the immune cells and having a similar Cancer Cell die in front of him he befriended. In the Live Action film he is even more tragic as that depiction has him condemned at birth and was White Blood Cell’s brother in that depiction.
He is also a Villain by Proxy since he never choice to be evil and was only due to his mutation causing him to do his actions plus the immune cells hunting him down.
He is also genuinely affable to White Blood, Killer T-Cell/Memory T-Cell and Natural Killer Cell despite his twisted personality.
He is also sympathized by the narrative and in-universe where his deaths are played for sympathy with his first two deaths in the Anime & Manga where White Blood Cell and other cells show some pity and remorse ia bit before Cancer Cell died in his second death and in Live-Action film dying in his brother’s arms.
Steroids
It has severe moral agency issues it is programed to do it’s job to suppress immune responses also making it a well intended extremist and Villain by Proxy too.
It is also Insufficiently Characterized as while it has a few lines all it says targeting and having a nice day being it’s only lines not much else is it motivation.
Verdict
A easy yes to both.