I was actually thinking that. In 2 and 4 she was always the pro who focused on getting the job done over personal feelings. Though she's generally helpful to Leon and obviously does care about him, she has no problems using him to accomplish her own goals.
I never really thought she had any real problems with bioweapon development/use in general given who she was willing to work for. To an extent I understand what she did in 6 though, since Simmons and Carla started the ball rolling and from her perspective she was probably just cleaning up loose ends and protecting herself.
I don't know about 'too nice'. She always had her own agenda.
Sally Cahill made Ada sound more caring due to her softer tones when speaking, especially when she says "...Leon".
In RE6, Courtenay Taylor gives a totally different performance, making Ada sound a lot more smug and cocky, especially with that new smirk she does. --- http://25.media.tumblr.com/tumblr_mdnau3FARq1rkuymjo8_250.gif
In RE6 she is cleaning her name, killing off her clone and used the opportunity to making herself pass as "Dead" when the Carla was reported as that by Chris and the BSAA.
Also she used the oportunity to get ridd of Simmons and his family so it was pretty much personal just like always, she just helped here and there whenever she had the chance --- "There is no knowledge that is not power"
In RE4 she was only doing things for herself but in 6 she is helpful to mankind, what's up with that?
reminded me of this............
on topic, thats Leon's effect. Think it was in Umbrella chronicles where she says Meeting with Leon changed me --- Those who break rules are trash but those who abandon their friends are worse than trash.