Just my interpretation, of course! But it shows just how powerful Giles is (we also see it when he challenges Willow in season 6).
It's meant to be agonizing, I think. What Giles did is out of character, so a slowwr deather shows how momentous Giles' action is.
Giles was powerful enough to kill Glory, just sayin'. Not in the way you mean, but he did it and that's what counts, right? I think killing gods/goddesses is damn near impossible, so Giles did it the only way they could: by killing Ben. Go, Giles!
Yeah, I was confused by cancer myself...but Xander is the better character, by far!
None of these - Giles, Buffy, Spike would be my top three.