"Instantly, the priestess changed into a monstrous goblin-spider and the warrior found himself caught fast in her web." - The Goblin Spider
"Tarantella" is based on "The Goblin Spider", an ancient Japanese short story. According to the tale, there used to be many goblin-spiders in Japan. During the daytime, they looked just like common spiders; but very late at night, they transformed into huge spiders and were capable of awful things. Goblin-spiders also had the magical power of taking human shape so as to deceive people.
While Spinnetod Lena does morph into a huge spider creature that ruthlessly kills its victims, unlike the Goblin Spider, she feels guilty for her crimes. The Spinnetod is humanized; while she doesn't want to blindly kill, her nature makes it nearly impossible to restrain herself. The Grimm writers also raised the stakes by introducing the rapid aging curse, making the decision for the Spinnetod even harder.