If she hadn't of killed _____ POSSIBLE SPOILERS
I believe that if JK hadn't of killed Sirius, she would have killed Dumbledore. (Sorry to any Dumbledore fans). There would have been a hufe uproar with a new headmaster / mistress and Dumbledore is the only one Voldemort was afraid of. For this reason I believe that.
Also, she could have killed a sudden wave of lesser characters. This would have made an impact in the way that Voldemort has arisen again.
On the whole, I was slightly disappointed at the 5th book, but liked it all the same.
Tom Felton Rulez
Sazii
FB MC