Hrm. A note about the above:
Nikita does not, in fact, waste any more than one police man. She's pretty much in a daze for the majority of the shoot out, and only takes down on cop, and it's not even certain if she realizes she's doing it. Her friends are the ones who take out the rest of the folks involved.
To continue, my take on the movie is as follows:
I do not, generally, see Nikita as a ravening sociopath on the lookout for new experience.
She generally comes across far more as a street punk/junkie who has no appreciation of life, of reason, etc., until she's put into the assassin program. There, in the midst of the training, she's presented with the cold fact that her life is now owned, that she can be destroyed at any time, and that she actually has to live up to a standard for once.
And she begins to care. She becomes professional. She even, upon being released into the world, begins to care about having a life (and a boyfriend)...but always aware of the hollowness behind it all precisely because she is owned by her employers.
And where her teacher formerly saw merely a psychopathic street waif, he now sees a young woman, full of life and potential. And has to decide how to react to it.
All of Luc Besson's movies include some measure of personal change for their main characters. Corbin Dallas learns to love, Leon comes to care about something more than his plant and his professionalism, Joan of Arc learns to face her issues of vengeance and accept...Nikita learns to respect life, and to need to have a life, to have reason, to have love.