I have an idea for an anti-heroic sci-fi character whose character arc runs from spoilt rich girl, to a refugee in the rubble of human civilisation after an alien invasion, to crewmember and then captain of a pirate spacecraft, and ultimately to empress of all mankind.
Running through all of this, I have the knowledge that this character is a successful sociopath. She doesn't care even slightly about the people around her, but she is highly intelligent and self-disciplined, perhaps unusually for sociopaths, and is good at pretending to care, yet when circumstances allow, she can revel in her ability to kill, maim, torture (both mental and physical torture) or otherwise discard people who are of no further use to her or who have become inconvenient, with the justification that the situation made it necessary.
Now, I have the idea to conceal from the readers - at least until the end - the detail that this character is in fact a sociopath.
In the end, I am writing about the first Empress of Mankind in a fairly realistic, gritty style, and she is no saint. She kills people - or uses them and disposes of them in non-lethal ways - whenever she can justify doing so according to the persona that she tries to project, that of a "nice girl" when amongst regular people. The pirate crew she joins gives her the opportunity to act more as she chooses than she can when amongst law-abiding people. While ultimately mankind ends up in a better state because of her presence, that isn't her goal, but a side-effect of her tactic to remain popular and to not be regarded as someone who is dangerous and who should be avoided or eliminated.
How could I best write about this protagonist without giving away the fact that she is a sociopath, and only ever pretends to care about others in order to gain sympathy and co-operation... and should I want to conceal this fact?