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 :-) )