Who is he?[]
Dark Danny the main antagonist of the television movie The Ultimate Enemy of the Danny Phantom series. He was created when Vlad Plasmius, Danny's archenemy removed his ghost half and Danny's alter ego merged with him. He then went on a ten year reign of terror before going back in time to ensure that he was created.
Reason for his removal?[]
Mainly, it's moral agency. Ghosts in Danny Phantom clearly have it so don't try to use that as a "gotcha!" With DD, the television movie makes it clear that Vlad's evil side overpowered Danny's ghost half meaning that him becoming evil wasn't his own choice in the matter. He is even shown being in pain as he was being transformed once the two merged. The show makes it clear too that after Danny lost his family and friends, he wanted that pain to be removed from him; ergo, his ghost half did not have his human emotions.
So basically, Danny thought that his ghost half was responsible for the deaths of his loved ones and did not want to feel that pain anymore by having his ghost powers removed. However, this caused his ghost half to not be attached to his human conscience. He was then made evil once more not out of any choice of his own.
But it should also be noted that DD was not the absolute worst that the show had to offer: other antagonists also threatened the world at large or threatened Danny's loved ones. Worse for DD, he doesn't have a confirmed body count. Sure you could argue that the desolation of the city would be an indicator that many people died, but that would be really stretching. Really when you take into account how characters like Vlad threatened the world, DD doesn't stick out much.