Due to the ferocity that Henry possessed as he hunted the monsters down, at that moment, the energy level that was in his body actually began rising. Despite the fact that the rate at which it was increasing was extremely low, but even then, progress was still progress.
One had to know that Henry had been hunting monsters in the dungeon for an entire week, but even then, there was no change to the energy reserves. But right now, with the number of monsters that he was killing increasing, there was actually a change that was happening. And for that, Henry was even more motivated, killing monsters left and right.
Due to the speed that he was killing the monsters with, the speed at which the monsters in the dungeon was being spawned could not catch up with the rate at which Henry was eliminating the monsters. And for that reason, Henry could only wait until a monster was spawned before he killed it off.