For the same reason she saved someone the first time, she has a kind heart.
Because she's a good person?
These. Also, it's the modern era...far less likely for humans to go on a monster hunt now a days. Most would just disbelieve him if he told his story about being saved by a monster spider lady. So, she doesn't really have anything to fear in saving someone nowadays.
That said...really surprised she didn't grow up hating humans. Then again, seems like a huge passage of time between then and now; so, perhaps she already went through her resentment phase toward humans long ago(would explain her attitude to the MC after saving him, though).