Master Gray was lucky to survive that attack of Roy, and so was Carl.
Heck, if half of the might of the Roar of the Sword hadn't been sapped away in its struggle against the palm and Master Gray wasn't an advanced-level Weapon Master, it would have chopped him, and then his son in half.
Seeing the youngster using a single move to defeat their master, the crowd revealed utter shock as they scurried away, not daring to stay close by in fear of getting embroiled in their battle.
Master Gray fell to the ground with his son.
If Carl weren't protected, he would've died.
Roy didn't want to become a murderer.
He had read their background and knew that the tailor cherished his shameless son so much that he would not hesitate to sacrifice himself for Carl.
That's the only reason why he didn't pull back punches just then.