I'd say that one was due to the stupidity of thinking they could make a chimp into a human, even if they did rescue it.
I stand by my reasoning, animals are not human - we shouldn't apply human morals to them and shouldn't think we have the right to judge them.
I don't agree with the killing of anything if it can possibly be prevented - in this case it could be removed to a safe environment - and can't cause future harm. Why do more - the only reason I can see is that you wish to see Justice done to an animal that doesn't understand the concept.
This isn't to say that I am against killing, but I don't like it to be unecessary. I eat meat (mostly because if everyone stopped thousands of cows, pigs and the like would just be put down... pointlessly), I would kill an animal or human if it threatened my life directly and there was no other reasonable solution. I would put down a pet if it had no chance of living a reasonable life (in constant pain etc), but I do not hold with killing if it serves no real purpose and can be avoided.
EDIT: and in the above, I beleive the chimps were shot during the attack, as the keepers could not see any other way. A move I agree with.