Still, to be blunt...considering how reluctant we seem to be to accept a redemption arc for Ward...would a "switch to the good guys" arc for Raina be any more believable?
Well, I don't recall her killing anyone in cold blood, which seems to be the major hangup with Ward. She's clearly on the side of science rather than any personal scheme.
Well, she never wielded the actual weapon, sure...but frankly, a lot of her actions throughout the first season were pretty much all-but-direct acts of cold-blooded murder. Kidnapping people, subjecting them to mental and psychological torment (and sometimes physical), subjecting them to mind control and mind-breaking...yeah, that's pretty dark.
I mean, Hand and Koenig were just killed. Raina's victims oftentimes suffered through things that would make them beg for death.
Plus, she noted that "her serum" was pumping through Garret, the same serum that caused one person (at least) to blow up. Granted, Extremis was developed by someone else, but she knew the consequences before she added it to the centipede project.
With that said , I do agree her hands are dirty, but I don't think her hands are as dirty as Wards. Betrayal is harded to forgive IMO.