The free rat, occasionally hearing distress calls from its compatriot, learned to open the cage and did so with greater efficiency over time. It would release the other animal even if there wasn’t the payoff of a reunion with it. Astonishingly, if given access to a small hoard of chocolate chips, the free rat would usually save at least one treat for the captive — which is a lot to expect of a rat.

What does it mean? That empathy is instinctual?

It suggests that.  

I wonder what drives humans to acts of evil if our primal instinct is to protect one another.  Is it a social pathology? 

Yes, this is a very interesting and complex subject. Our brains are wired for empathy, and we have the instinct to protect one another. But our brains are also wired for survival so we have the instinct to protect ourselves (the lizard brain that instigates the fight or flight reaction). Sometimes we act on the empathy instinct, and sometimes we act on the fight/flight instinct. The fascinating thing is that no one is the same person all of the time - it depends on context. The same person who is very empathetic in one context can act totally different in another context.

Check out Philip Zimbardo's work on the Stanford Prison Experiment for a fascinating (and somewhat disturbing) example of that.

That's so true that context matters, given that we have the capacity for both.

Yeah!  they give me faith in human nature, uh, wait, I mean rat nature, but still this must mean there's hope for humans ;)

There is hope for humans. Especially the ones who emphasize the empathy instinct.