@Doodmandood
Well she thought they were burning her nest because they were trying to hurt her, wich put Kumo-chan in a defensive stance and agressive mood, and even the first attack from Kumo-chan didn't happened untill the humans attacked her first after their encouter.
So I would say it was a missunderstanding from both parts, Kumo-chan because she tought they were there to hurt her and the humans because they got scared and attacked her when she took an agressive stance.
Yeah her motivation to kill all of them afterwards was for the XP as it was not needed to ensure her safety but at that point what diference would that make?