- If Ada had anything to do with Leon other than aforementioned, she would have left the bad side and joined forces with him, even as far as ask him to just spend life with her in seclusion.
Firstly, she saves Leon despite the fact that he was nothing but a hindrance to her mission. You'd have to be emotionally disabled to miss the love interest between them.
Secondly, Ada's motives and/or personal reasons are unclear. She isn't on the "bad side", she is portrayed as an anti-hero, not an antagonist. Since she loves Leon she would outright confess it and live a happy life with him in seclusion? She likely has personal reasons for working with the organization that have nothing to do with Leon. Stop being naive.
more capable and bound to blow her brains out.
Only capable of killing idiotic villains who wear shades in the dark.