The real Long Chen was standing right beside the man he killed.
The biggest limitation of Long Chen's Illusion was that he had to keep his focus on the person he wanted to cast his Illusion on. If he couldn't see the man, his Illusion wouldn't work, that's why he stopped mid-way so that the others would land in front of him.
He didn't want a situation where he was fighting one and the others were fighting another which would make it difficult for him to cast his Illusion on everyone.
Fortunately, the 5 red-robed cultivators stood didn't beside each other.
Long Chen chose the one who was standing in the corner to kill so that he could keep his eyes on everyone.
As the 5th cultivator died, his sound did alert the others, but before they could react, Long Chen had managed to slice the head of another one.
In an instant, 5 red-robed cultivators had become 3, though they were more alert now.
