So, mine isn't a fictional being he/she was a real person many years ago. However, I would like to say before hand that I do not at all agree with the methods which where used in attempt to achieve his or her goal.
But, Jack the Ripper
For the fact that his or her goal, which the gender and identity of this serial killer was never truly discovered, wad to rid the streets of "ladies of the night" or prostitutes if you will. The methods where sick and completely wrong but the goal was noble.