This is probably left of the field but some thoughts that came to mind while reading this:
We don't need to believe in compassion in order for it to manifest in action. I recall our middle kidling when not yet 2, he was in my arms while walking through a flower shop. We came to a fountain feature with fake smoke coming out. I put my hand in to hear him exclaim "oh no Dad! Hot!" I am sure he was not aware of compassion whether by name or by reputation.
So it simply manifests, a product of our kamma which includes genetics, evolution and probably a bit more than that. What is it? It is a potential, somewhat like our potential to learn a language which we all possess. In the absence of a language around us we wouldn't learn it, but the potential is there.
An aside in response to Kim.
Kim wrote:If I refuse to recognise my evil side, refuse to feed its impulses by acting on them, they will dwindle away. So we are taught.
Similarly, if I refuse to acknowledge the existence of compassion, it will dwindle away. In such ways do human monsters (Hannibal Lectors, for instance) create themselves.
Not sure if I am reading Kim correctly but not recognizing the "evil side" is exactly the culprit. Facing all of it including the stuff we don't want to see is how we learn to become fully human. Not acting on it, not feeding it, is of course a good idea, but face it we must. Otherwise it will pull the strings quietly and just lie in wait until the opportunity arises (remember Death and the Maiden for instance or Dr Mengele, who was a war hero saving people from a burning tank only to be corrupted by the unlimited power he held over other human beings and his formerly unacknowledged dark impulses).
Kim wrote:Passengers are herded towards the Gates of Hell, weeping and wailing ... until one guy says, 'this is totally unbelievable' and manages to laugh at it, whereupon Hell falls to pieces in front of their eyes and everyone finds themselves back on the train and, soon, pulling into London Central Station. So far so good, but the same guy looks at the grand public buildings of London and starts to laugh again ... and London starts falling to bits in the same way.)
Of course it does - there is no London Central Station (literally, not just in the emptiness sense)
Returning to the OP:
Death: YOU NEED TO BELIEVE IN THINGS THAT AREN'T TRUE. HOW ELSE CAN THEY BECOME?
It seems to me they don't need to become, they can just manifest. Sure giving them a name or even a notion of "them" is already believing in them, is this necessary? Our kid didn't have to believe in compassion in order to demonstrate it. Indeed thinking about it can sometimes get in the way. On the other hand cultivating it can also be useful up to a point, but then this crutch is also to be thrown away.