The builder can't move through a sword. When you stick your sword into the path that the builder has to follow, it breaks the path and the builder will turn around and head back to the...thingy.
If this still doesn't make sense, try this experiment out:
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After you step onto the pressure plate, move west to lure the roach down towards you. While the roach is moving, move east and bump NE. When the roach gets close enough to activate your sword, head around the circle and jump into the doorway at the top. This ought to force the builder to turn around for the first time. Repeat as necessary.
Josh
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"
Rings and knots of joy and grief, all interlaced and locking."
--William Buck