Blood Forest
The castle was huge, and there was a lot of ground to cover. On top of that, Quinn wanted to attack before they could react. Thus, he needed no distractions for his plan to work, which was why he was running around, defeating anyone he could find.
He wouldn't outright kill them and just needed to injure their bodies to an extent where they would no longer get up, but he had to use his speed to the max to run faster than he usually could.
One way to do this was to use the Blue fang set, but that only worked for a limited amount of time, and on top of that, this was Quinn's trump card. It was a very special set that got him through very dire circumstances, so instead of using it from the get-go, he came up with a new technique.