The first time the Black Phantom left a mark, he did it with a mere speck of his power. So it was expected to be a mere mark. But the second time, he did it with his real body. And that wasn't just a mark.
When the Black Phantom saw the grand prize William prepared for him, just in short time after meeting him for the first time, he decided to grant him a speck of his power.
It wasn't as if William was alien to such power, after all the Devour ability he had originally came from the Black Phantom. So William never noticed the difference, never noticed that something was off in his and his monstrous weapons' techniques.
The techniques all had the vibe of the Black Phantom spirit, the scary spirit that was enough to scare any other spirit or life form by mentioning his name. So such a response from everyone, be it allies or enemies, was quite expected.
