This was William's plan.
After the first exchange, when the Hand Axe chopped into Gatekeeper Pim's shoulder, William drew two conclusions.
1. Even "Deathlike's Dark Hand," which met the Knight Order standard, when deeply buried in Pim's body, only reduced his epidemic value by 1/10.
Such a perfect opportunity to attack happened just once; getting another chance to strike in this manner during the follow-up fight was almost impossible.
It was concluded that it was not possible to defeat such a Skin Messenger across tiers.
Even if he were to win by an extremely small chance, William would pay a grievous price, and then he would be taken like fish meat into the wholly unknown depths of the Skin Factory without a sliver of chance to resist.