Essentially, there have been two Negaducks in the adventures of Darkwing Duck. The first was purely made of Negatrons (who has not appeared in comics) and the other was an alternate dimensional Darkwing.
First: Megavolt's Negaduck
When Megavolt was robbing a bank, he shot Darkwing with his Tronsplitter, which split Darkwing into two beings. One was made entirely of Positrons (named Posiduck) and the other was made entirely of Negatrons (named Negaduck). Long story short, Gosalyn and Launchpad were able to use the tronsplitter to put them back together to get the regular (and we use that term loosely) Darkwing back. They would later use the tronspliter again to defeat the regular Negaduck. This version has not appeared in comics.
Second: Whats his name?
This Negaduck is from the Negaverse, where everything is opposite. Much like what Ultraman is to Superman, Negaduck is the complete opposite of Darkwing Duck. Negaduck is a thief, the ruler of his St. Canard and a...down right bad guy! Oh! and he has a Chainsaw!
Negaduck (both of them) was created for the TV show Darkwing Duck. He currently appears in Boom! Studios' new series by the same name.
The Duck Knight Returns
When Negaduck was taking his laundry to the worst dry cleaner in town, he saw Launchpad with two pairs of clothes. One was the "pathetic purple version" of his own duds and the other was an ugly green sweater vest and a pink shirt. So, Negaduck followed the "big galoot of a sidekick" back to the Mallard Residence. There, he watched the house for months.
Finally, after a good run (with lots of loot) with the Fearsome five (which he was then leader of), Negaduck was ready. When Quackerjack found out Negaduck was going to attack where Darkwing lived, he begged Negaduck to bring him along. Negaduck told him that he was "not mean enough" and ripped up his doll, Mr. Banana Brains and leaves.
He attacks the house hold using a tank. Drake was helpless against his arch rival. He was saved by the metal hands of the Quackwerks Crimebots. This moment of helplessness would scare off Darkwing from the superhero business, but this is not the end!
Crisis on Infinite Darkwings
After Negaduck was placed in the Quackwerks jail by the Crimebots, he was saved by the notorious sorceress known as Magica De Spell. A team up was initiated and the plan was set in motion. Now that Darkwing was back in town, Negaduck had determined he had to darken Darkwing's image. Using his own knowledge of dimension jumping and Magica's er...magic, they were able to kidnap and brain wash alternate versions of the Duck Knight. He took these Darkwings of the Multiverse and used them to frame Darkwing for crimes he didn't commit. Ultimately, Negaduck unleashed hundreds of alternates upon the city. To make matters worse for Darkwing, an ancient evil was manipulating the waters of underneath the Audubon Bay Bridge.
Darkwing and friends were able to defeat the combined power of Negaduck and the ancient evil by using Megavolt's Tronsplitter and the power of love (no! seriously! Its true!).
Currently, Negaduck is divided into thousands of pieces, but the bad ones always return so we'll be seeing him very soon!