Aisha doesn't like face-to-face fighting, she's very much not a fighter. She's a killer and gets her thrills from a clean, successful assassination.

I could say she was former a hunter who simply started to grow a taste for huntin people instead of animals. That would give her the sneaking, marksmen, and knife skills for an assassin. She'd still live out in the wilderness instead of the imperial city so she can hone her alchemy. Then she would chose to travel around the country as a traveling merchant as a cover for her assassination tendencies.