Your problem is that it wasn't a meaningful or substantial sacrifice. As that spider's death was guaranteed anyway, and you kill spiders all the time, how was this, in anyway, a worthy sacrifice for your "spell?" The backlash on you was probably due to such a petty excuse of a sacrifice. Should have went with the neighbour's puppy instead, or perhaps something even more substantial, such as a person. Maybe how pure the soul is makes a difference as well? The puppy is again a good idea, but if it has to be a person maybe you should sacrifice a baby.