How is Ludo Bagman a 'bad wizard'?

Question

In Harry Potter and the Goblet of Fire, Harry, Ron and Hermione run into Dobby and Winky at the Hogwarts kitchens. When Hermione mentions Bagman is a judge for the Triwizard Tournament, Winky (Crouch's sacked house elf) says this:

“Mr. Bagman comes too?” squeaked Winky, and to Harry’s great surprise (and Ron’s and Hermione’s too, by the looks on their faces), she looked angry again. “Mr. Bagman is a bad wizard! A very bad wizard! My master isn’t liking him, oh no, not at all!”
“Bagman — bad?” said Harry.
“Oh yes,” Winky said, nodding her head furiously. “My master is telling Winky some things! But Winky is not saying . . . Winky — Winky keeps her master’s secrets. . . .”
_{Harry Potter and the Goblet of Fire, Chapter 21: The House Elf Liberation Front}

My question is,

Why is Bagman considered to be a 'very bad wizard'?

Yes, that is only the opinion of Barty Crouch Sr. Everyone else seems to like Bagman (except Percy Weasley, of course). But even Percy doesn't seem to think Bagman is a bad wizard. Percy just doesn't seem to like that Bagman is not a very good Head of Department because he hasn't sent out a search party for a missing Bertha Jorkins. Bagman is also shown (in various parts of the book) to not really care about anti-Muggle security

"... And Ludo Bagman’s not helping. Trotting around talking about Bludgers and Quaffles at the top of his voice, not a worry about anti-Muggle security..."
"I thought Mr. Bagman was Head of Magical Games and Sports,” said Ginny, looking surprised. “He should know better than to talk about Bludgers near Muggles, shouldn’t he?”
“He should,” said Mr. Weasley, smiling, and leading them through the gates into the campsite, “but Ludo’s always been a bit . . . well . . . lax about security..."
...
Ludo Bagman was easily the most noticeable person Harry had seen so far, even including old Archie in his flowered nightdress. He was wearing long Quidditch robes in thick horizontal stripes of bright yellow and black.
_{Harry Potter and the Goblet of Fire, Chapter 7: Bagman and Crouch}

Bagman walks about the campsites outside the Quidditch stadium wearing his Quidditch robes while all other Ministry wizards are in Muggle clothing.

He is also quite unscrupulous, paying off his betting debts with Leprechaun Gold (which disappears after a few hours), and betting on Harry in the Triwizard Tournament even though he is a judge himself.

However, as much as Barty Crouch Sr. loves sticking by rules and regulations, these shortcomings of Bagman's can hardly classify him into the 'very bad wizard' category. Winky's demeanour seems to suggest Bagman is almost a dark wizard. Is there any canon evidence (movies don't count as canon) which corroborates Winky's statement? If not canon, well reasoned answers will be appreciated.

Answer 1

Crouch believed that Bagman was working with the Death Eaters

Barty Crouch had prosecuted Bagman during the First Wizarding War for providing information to Augustus Rookwood. Crouch was notoriously draconian, and probably believed Bagman to be guilty.

“Ludo Bagman, you have been brought here in front of the Council of Magical Law to answer charges relating to the activities of the Death Eaters,” said Mr. Crouch. “We have heard the evidence against you, and are about to reach our verdict. Do you have anything to add to your testimony before we pronounce judgment?”

—Harry Potter and the Goblet of Fire

Bagman, however, had likely merely been misled by a true Death Eater, Rookwood.

“But I’ve told you, I had no idea!” Bagman called earnestly over the crowd’s babble, his round blue eyes widening. “None at all! Old Rookwood was a friend of my dad’s . . . never crossed my mind he was in with You-Know-Who! I thought I was collecting information for our side! And Rookwood kept talking about getting me a job in the Ministry later on . . . once my Quidditch days are over, you know . . . I mean, I can’t keep getting hit by Bludgers for the rest of my life, can I?”

—Harry Potter and the Goblet of Fire

Crouch presumably told Winky that Bagman was working for Voldemort. This was probably the information she was referencing when she claimed Bagman was "a bad wizard."

Crouch undoubtedly believed that anyone he prosecuted was guilty, and indeed had presided over the trial of at least one other innocent, Sirius Black.

“Despicable,” Mr. Crouch spat at Dumbledore, sitting down as Bagman walked out of the dungeon. “Rookwood get him a job indeed. . . . The day Ludo Bagman joins us will be a sad day indeed for the Ministry. . . .”

—Harry Potter and the Goblet of Fire