This is why they needed each other.  (generalizing big time here)  Guns was big because the guys loved the heavy sound and bad ass lyrics and women loved the poppy songs and SCOM type lyrics and melodies.  They had to coexist to reach the level they did.  Axl's version would have been more mainstream and that would have never reached the levels they did.  Slash's version would have been an Aerosmith knockoff and that wouldn't have made it big either.  What it really comes down to is the same thing that made them great tore them apart.  It was inevitable regardless of who was more responsible for it.

That doesn't mean it wasn't his fault.  It can be someone's fault for being themself.  Sometimes in life you decide to stick with someone or a relationship that you know isn't good for you because you don't know what else to do.  Eventually you figure out how bad it really is and you move on.  Nothing changed other than one person realized that whatever good came wasn't worth the bad that came with it.

Fault is really the wrong word - I think Axl is most responsible for it, but it isn't necessarily his fault.  Slash knew that if the band was going to be huge, they needed Axl, so they put up with a lot of shit that they didn't necessarily want to because they felt they had to including signing over the rights to the name.  Eventually they decided putting up with Axl wasn't worth it.  Axl didn't change, but it was his actions/decisions that are mainly responsible for what happened.

I don't think there's much question that Axl can be an asshole and he's difficult to work with.  From what I've read so far in Slash's book, he can be an asshole and difficult to work with as well.  I think most really talented people are assholes and a lot of that comes from expecting others to be at the same level that they are at.  This is why in sports, most great players don't make good coaches.  They expect more from guys than they are capable of doing.

Going with this line of thinking, it's certainly possible that Axl had high expectations regarding his vision of what the band should be and Slash had high expectations of his ideas of what they should be.  They co-existed because AFD is much closer to Slash's vision than it is to Axl's and Slash gave in somewhat with UYI, but after that neither side would give at all.

I can't believe I'm going to say this, but after typing this out, Slash comes off as being a spoiled brat.  Thinking back to the book, there are multiple examples of him just turning to drugs when he didn't get his way.  I still think he's great, but he's not as different from Axl as I think he'd like to believe.

Axl wrote some of the most troubling lyrics ever (in mainstream at least).  I hardly think he can judge someone else as not writing happy songs.

Slash wrote what he felt.  I don't imagine he was very happy at the time.  He was part of this great band that was falling apart and he wasn't strong enough to do what it took to hold it together.  I wouldn't be writing "Shiney, Happy People" at that point either.