I've been thinking about stories and films with tough, anti-social MCs. About the problem of keeping a reader interested in some-one who's angry a lot of the time. Or a loner. Especially when that some-one is a woman over 30.
I've been thinking especially about Smilla's Feeling for Snow. Smilla is prickly, angry, bitter, tough as nails. But there's also this hurt, soft side, a side she reveals to her neighbour's child. Her anger comes from being dislocated and stuck between cultures, from feeling orphaned by a distant father who tore Smilla away from her mother's family. The first chapter ends with her resolve not to desert Isaiah after he dies a suspicious death. Smilla is likeable because she's out to avenge an innocent child. And because she's also a victim herself.
It's the same with the Sven Tveskoeg character in the David Gunn books. And the 'preventer' character played by Liam Neeson in the film Taken. Both ruthless bastards, but they're on the side of the weak and vulnerable. You cringe as you watch Liam Neeson torture the bad guy, but he's a father; you understand he needs to do anything that'll get his daughter back.
1st step - ask hundreds of questions, learn everything I can about my female assassin.