Descending
Since the caves had collapsed in many places, William and Carl's descent was greatly slowed the deeper that they went.
Their rate of movement was slowed even further when William's Illusionist's Cloak detected multiple Cultivators tunneling through the cave behind them. It wouldn't be long before the other thieves caught up.
William was forced to set traps to deal with the pursuers, consisting mostly of the Ice and Time Elements to slow them down as much as possible. While this was mostly effective, it would only stop the weak ones. The Nascent Soul and Golden Core thieves, however, needed but a few minutes to break through.
William took the initiative to kill whatever thieves they came across, not worried about the ever-diminishing Humanity Points when achieving a sneak-attack.