Why not kill animals to hasten their rebirth? This may be primarily for the Tibetan style practitioners. Also, my question assumes that "rebirth" etc are not metaphors. 

Since non-human animal existence is lower down in the six realms, why wouldn't killing them (painlessly) be considered a good thing, since it then allows the being rebirth and maybe it will be reborn as a human? For example, in "Seven Years in Tibet", earthworms were saved from death. Isn't that in some sense cruel, since saving an animal prolongs the time it has to spend as an animal?

I think I know the answer; namely, that karmic effects need to be worked out and if we kill an animal prior to that point, then it's simply going to be reborn as another animal until it's paid whatever "karmic debt" is involved. Therefore, killing an animal doesn't serve to give it a chance of a higher rebirth.

But that's a guess. Any ideas? (And again, I know some people think the whole "rebirth" thing is just a metaphor. Fine. But this question is predicated on that not being the case, so in this particular case, please keep yer metaphors to yerself :-) )
What makes you think that they will even be born at least as animals again? It's very difficult to get into higher realms once you go down. An animal can very rarely have a thought devoid of craving/lust, aversion and ignorance. So whether you kill an animal or let it die naturally, there's very little chance for it to get a better birth. There's a reason why the hells are called the great houses of Sansara. Once a being sinks below the human realm, it usually keeps sinking further down as it'll be mostly creating bad karma. Unless some good deed from a past life comes forward at the time of death, there's very little chance for it to escape hells. There are also rare instances of some animals listening to the Dhamma and being able to be born in higher realms. So if you are really interested in helping them, make them listen to the Dhamma and hope at least 1 in a million would be able to make use of it. 

Breaking the 1st precept with such a false belief would probably make you join them instead of saving them :)