I have never been able to figure this out.  What was it that killed sg-1?  Was it Cam, Vala, or the new Genrel?  Or was it the new enemies?  Or was the show just getting to old?  In my opinion I am mainly leaning on Cam and the new enemies.  There were a lot of changes that might have lead up to it, any ideas?

I've always assumed it was simply a combination of all of the above. If things had changed one by one then the series might have coped but even then I don't think that would be guaranteed. It's quite possible that Sci-Fi simply wanted the slot for somethif else.

it died because it should have finished after season 8.

Season 8 wraps up pretty much every plot line and the goals of the SGC, and others, are achieved.
9 and 10 are just extra season to make a few extra bucks off the stargate name.
they are bad overall and personally, i wouldnt recommend them to people.

I would'nt say seasons 9 and 10 were bad, just that after season 8 it started to slope back down again rather that continuing upwards (if you get my meaning?)

I loved the Arthurian plot of Season 9 personally and i just think that they wanted to make space for SGU and Atlantis