User blog comment:Zathronas/Speculative Author - RWBY Relativity/@comment-25396609-20160225025456

Just no to the thing about Salem fusing grimm into herself.

Her connection with them is clearly a far deeper one (and less stupid) than that. It is more likely she is either their progenitor, or an evolution. A grimm which has long since crossed the bishounen line.

But humans fusing with grimm is some straight mary sue bullshit and i pray to god RT aren't stupid enough to try and pull that.

My own theory on why grimm attack humans is similar but distinct from your own, and a bit more scientific.

I have, in the past, proposed that souls, aura, and semblance are emergent properties of a sufficiently complex neural net. Simply put; the soul is the expression of the electrical impulses in the brain and nervous system, aura is a projection or extension of that energy (still not 100% on that part, need more information) while Semblance is a property of aura that is derived directly from the neural net.

Specifically, the semblance is the manifestation of the unique patterns within a persons body, granting their aura an ability that distinguishes it from all others.

Grimm tie into this on a very fundamental level; i believe that the universe RWBY takes place in is inherently averse to complex intelligence, and as such entities were created as a byproduct of the emergence of such a property, and they seek to destroy something that reality itself is displeased with.

They do not attack animals who possess souls because, though they posses a neural net capable of generating the phenomena known as aura, their minds are not intricate or unique enough to produce an adverse reaction.

The distinction must be made between intelligence and sapience at this point. Intelligence is the result of a neural net able to produce aura, while sapience is the cause of individuality, and as a result semblance.

tl;dr: all souls generate aura, not all aura has a semblance, and grimm are only interested in the ones that have semblance.