Sith are Sith and Jedi are Jedi because of they followed a strict and demanding training program. The training is based on thousands of years of gained knowledge pasted on from Jedi to Jedi or Sith to Sith.
Someone like Ventress is like have a person who has gone through medial training like EMT, Nursing, etc.. and then buys a lab coat and calls themselves a doctor. Sure they have a stethoscope, a little hammer, and some tongue depressors, but they are no doctor.
Now there could always be something like a Jedi who believed the way of the Jedi was not the best way and not the dark side either. They could come develop their own teachings that reflected their beliefs. Much like how the Lutherans developed from Catholicism. Or the way
Edward William Barton-Wright studied
jujutsu in
Japan and creates
Bartitsu upon returning to England.