... the leopard sensed that something just wasn't right with the rat. For instance, dogs can sniff out cancerous tissue, and they can sense abnormal electrical (electromagnetic?) activity in epileptics about to have a seizure, too. So, given that even human observers noted the rat's obvious unusual behavior (i.e., it neither tried to scamper away nor play dead when confronted with an otherwise mortal enemy many times its own size), maybe kitty said something similar to a comment I made in another post elsewhere ... "Hey, this looks pretty good." + [Sniff] + "OMG ... Eeewww!" + "Dude, that's just E-V-I-L sick!"
I mean, perhaps like humans, some creatures might also be aware of the fact that what one eats can become part of what one is. I don't have any references, but I remember reading several years ago that studies have shown that behaviors and perhaps even memories can be transferred from one animal (I think it was rats being studied, not sure) to another upon the one (rat) being fed and eating the brain of another (rat). So, yeah, it might be the case that the leopard noticed something whacko about that otherwise bite-sized snacko and therefore said "No thanks!" and just walked away.
To say the least, if one blinds oneself to its inclinations toward brutality, Animalia is a most curious place to explore. But, it is quite dangerous, too ... sadly human-like in many ways. Indeed, I think there's much the camera eye of humans tends to gloss over and not see, and lots the ear never hears, about either domain ... and the sometimes shocking relatedness 'tween the two.
Don (now disengages from theorizing how minds of a cartoonish cat and mouse might differ)