I the term "Dark Jedi" as understood now may be an oxymoron, but the original intention of the term was to describe Jedi who had fallen. The term Sith had not yet been devised. From Wikipedia:
Dark Jedi and the Sith:
Dark Jedi is the unofficial name given in the Star Wars universe to antihero fictional characters attuned to the Force and adept in its dark side. The concept of "Dark Jedi" is not endorsed anywhere within the movie trilogies. They exist by that name only in the Star Wars expanded universe, including video games such as Star Wars: Knights of the Old Republic and the Star Wars: Jedi Knight series; the term is never used in any of the six Star Wars films. So called Grey Jedi and Rogue Jedi are terms that refer to Jedi who don't follow the Jedi Code and/or the decisions of the Jedi Council, or who have defected from the Jedi Order and/or have rejected the Jedi Philosophical concepts of "Lightside" versus "Darkside". Fallen Jedi refers to former-Jedi-turned-Sith who were seduced by the Dark Side (such as Revan or Anakin Skywalker).
Dark Jedi and Grey Jedi are a label to a specific archetype of characters in the Star Wars Expanded Universe defined as Force-attuned beings that meet two requirements:
They do not hold themselves to the dogma of either the Jedi or the Sith.
They use their Force abilities for less than virtuous pursuits–acts that are normally forbidden by the Jedi Order.
Dark Jedi in the Expanded Universe typically refer to pupils of Luke Skywalker's Jedi Academy who have fallen victim to the dark side. Dark Jedi are also extremely common in the Knights of the Old Republic series in league with the Sith Empire and are often fallen Jedi from the Mandalorian Wars. Dark Jedi also appear in the Jedi Knight series of Star Wars video games, where Kyle Katarn uncovers Imperial plots linked to Dark Jedi. In Jedi Outcast, Katarn faces an entire army of Dark Jedi, cloned and lightly trained force users who follow the main antagonist. Dark Jedi have distanced from Sith in more modern Star Wars media. They are not sworn enemies of the Jedi Order and do not consider destroying it a priority as Sith do. Dark Jedi do not always appear in pairs with a master and apprentice and often work alone.
Because the term Sith was never spoken in the original trilogy (although Darth Vader was described as "Lord of the Sith" in the published screenplay), early Expanded Universe products usually considered the "evil Jedi," those who joined the dark side of the Force, as "Dark Jedi." In his novel series The Thrawn Trilogy, author Timothy Zahn labeled Sith Lord Darth Vader and Emperor Palpatine as Dark Jedi, and the term "Sith" was never mentioned in the series until later reprints of the novels.
So it was basically a generic term used to describe force users who didn't necessarily follow the Jedi or the "Sith" dogmas, and generally used the Force for less than good actions.
As for current Disney Canon
Here is the definition of a "Dark Jedi" from Leland Chee, the official keeper of Star Wars continuity:
A dark Jedi is anyone who practices the dark side of the Force whether or not they were a former Jedi.
As far as your exact question
So is there any level of canon where there are any Jedi that would actually fit this definition of a "Dark Jedi" - one who believes in the Jedi Code, but intentionally goes against it?
I don't believe there is. There are examples of Sith or Dark Jedi infiltrating the order for various reasons, both personal and in an attempt to bring both the Jedi and the Republic crashing down, but no examples I could find of a Jedi, deciding they no longer wanted to follow the Jedi Code but remaining in the Jedi Order, and now using the dark side of the Force.
As for Quinlan Vos as an example
According to Wookieepedia, (which is still based on Legends canon at this point)
He actually became a member of the Sith when he infiltrated the Jedi Order in an attempt to get his revenge, so he doesn't even technically qualify.
I am not aware of his Disney canon story enough to speak to whether that has changed or not.