The word "assassinatrix" has a very specific meaning: a female assassin. However, there are many other words that can be used to describe a woman who is skilled at taking lives. "Hitwoman" is perhaps the most common synonym, although it implies a certain degree of professionalism and hired contract work. "Murderess" is a more sinister term, suggesting that the woman in question takes pleasure in killing. Other options include "killer," "executioner," "slayer," and "eliminator." Of course, none of these words are particularly flattering or positive, but they accurately convey the idea of a woman who is proficient at ending lives.