The systems in place that penalize you for killing (or don't give you a bonus, in MGSV's case) suggest the "true" (read: canonical) BB/Venom don't kill unless absolutely necessary. They only kill when they have to for the sake of themselves or others. It speaks to their heroic quality and separates them from your run-of-the-mill coldblooded killer.I guess, but how is that different from what Quiet is doing? She takes out some people on her way to kill her target. That's basically what every protagonist does in a Metal Gear game. I didn't see her approaching the player slowly as some sort of sadistic act, I saw it as her approaching with caution because her target is a legendary soldier.
Obviously the real reason is that she does everything slowly to add dramatic tension, because if she actually acted like the professional assassin she's billed as, the game would have been over very quickly.
But Quiet doesn't care about being heroic. We don't know if she really cares about anyone beyond, well, Venom, and a brief display of concern for the child soldiers. With the material we have in the game, there's nothing to suggest she has a conscience prior to her arrival at Mother Base, which just makes her unsettling. I wish there would've been some insight into why she became a killer in the first place, something to make her a more sympathetic character.
Again, I know it sounds like I hate her. I don't. I actually think she's a really neat character in many ways. I just want more context for her.