Taking in fandoms one at a time.
Am a Resident Evil nut though.
Hmmm. Allow me to inject a few of my thoughts :)
Ada is a professional spy with a secret motive. From that fact alone, we could point out several things:
First, what she “sells” is her services alone, not particularly the plagas or anything like that. She doesn’t go to different laboratories on a whim, steals precious data and then puts them up for auction; rather, she was most likely ordered to do all that stuff. And this is seen in the Damnation movie, where Ada was ordered by someone to get a Plaga sample—to which, in the end, she was successful in acquiring and that everything went ‘according to plan’, with the exception of a hitch (a.k.a the international warrant) that could have been easily dealt with.
Did she care about how the clients used the data afterwards? Most likely not; she was a professional, and what lies outside the nature and specifics of her job was most likely none of her concern, unless given the motivation to.
This would then raise a point that ‘Ada works for the bad guys’. Well, she doesn’t work specifically for the bad guys alone; she’s a mercenary, and Ada, particularly, isn’t picky with her clients—UNLESS the said clients (or the job offerings of these people) came in contrast with her secret goal—which is unknown up until now.
And this follows up the second point in the statement: She has her own goal—her own secret motive. One that she would willingly put above all else and that could even warrant betrayal to her clients. This could be seen in RE:4 wherein Ada betrays Wesker (by not killing Leon, by killing Krauser, and by not handing over to him the true plaga samples) because of her own secret agendas and affiliations.
So more or less, Ada is a mercenary—a spy— with her own goal. I wouldn’t really count her on the villain list as much as I wouldn’t also count her on the heroes list. Her character-color lies outside the black or white spectrum; it is on the grey, neither “evil” nor pure good—not until her true motives be revealed, that is.
Well, her relationship with Leon can be a complicated case, and I would save it for another essay. But you could look at how the RE6 files worded it: They do not consider each other enemies. At least that much was given as to why Leon had not fired the bullet to Ada yet, nor why Ada hadn’t assassinated him given the countless times she could have done so.
And I, for one, wouldn’t want her to be killed, or at least not yet. She is by far one of the most interesting characters in the REverse (the most interesting for me, actually) and that killing her would warrant more unsolved questions than answers.
…And those are my thoughts :) Don’t worry, you’re not wrong or insane or anything—it’s just your own way of viewing things, that’s all.
(edited for the confusion: Simmons wasn’t the dude in the end of Damnation!)