Board Thread:General Discussion/@comment-3257939-20140105070225/@comment-5226885-20140114234932

So far, we only know that the soul is what generates aura and allows humans to fight off Grimm effectively. As this is an original work by Monty Oum, he could have it so that emotions and higher levels of intelligence are not necessarily connected to the soul. Therefore, Grimm could very well have emotions and be able to express confusion, like animals can in a way that humans don't truly understand. Plus, when Jaune said "That's why we fight them", Pyrrha responded with "It's not a question of why, but how." That could mean that the prescence of the soul is not the underlying reason for their conflict, though we do know that Grimm don't have souls and so can't use aura either at all or possibly as the humans of Remnant know it.

I believe that during Jaune's battle with the Ursa Major, the Grimm was more interested in the sap than with the humans, but fought Jaune as he was a threat and Jaune's attack ignited its instinct for fighting humans.