Why is the hornet so fatal to wasps?

Because it not only seizes the wasps while feeding, but hawks after them when on the wing. Having captured them, the first operation is to snip off the head, then to cut away the lower part by the waist. Sometimes you may hear the hornet shearing away the outer coat from a wasp's body, and crushing it with its strong mandibles ; sometimes devouring it, but generally only sucking the juices it contains.

Why are hornets supposed to fall victims to each other's voracity towards the end of autumn?

Because they fight desperately when they meet in pursuit of prey, biting each other's body, and trying to get their mandibles under the head of their opponents, to snip it off. Pairs of them often die after such a contest.