And here is a quote from the biography on allmusic written by Carl:
"After [...]Sympathy for the Devil," the band separated to take time away from one another. Thinking that Guns N' Roses was just taking a break, McKagan started working with Slash on the Slash's Snakepit project along with the other members of Guns N' Roses, except for Rose.
Sessions for a new Guns N' Roses album continually fell apart, and McKagan started working with the Neurotic Outsiders and several solo projects from fellow- and former-Guns N' Roses members. As the years began to tick away, Rose finally sent news to the remaining members of the band that they had to stop working on other projects or they were going to be fired. That was the final straw for McKagan, as he separated from the band and joined Seattle punk rockers Ten Minute Warning, a band he had played in almost 15 years before."
That doesn't make any sense. Duff was never really involved with Snakepit, he co-wrote one song but he didn't play on the album or tour with them. He was working on his own stuff at the time.
The whole time with Neurotic Outsiders he (and the rest of the band) said it was just a side-project for fun and never going to take time away from GNR. Neurotic Outsiders was basically the same thing as Camp Freddy - it was a jam band that played primarily covers (although they did obviously write some songs of their own), but it was never a serious thing.
And he didn't get involved in Ten Minute Warning, Loaded or any of his other projects until after
leaving GNR, so they can't have been a reason for him to quit.
From what we know Shelly's post pretty much covers it. I don't have a link here but look in any of the existing topics on why GNR split and you'll find a link to a site full of interviews where Duff discussed his reasons - essentially saying that GNR just wasn't what he wanted to be doing at that time. He didn't hate Axl, they were just moving in diffrent directions by that point.