I tend to read “categorical imperative” in the strongest, Kantian sense, an imperative statement that is a priori valid irrespective of context or reasoning—i.e. murder is just wrong and you just shouldn’t do it period.
If “murder”==”the wrong kind of killing” then “the wrong kind of killing is just wrong and you just shouldn’t do it period” is a tautology. It would seem you can get cheap categorical imperatives by jumping to tautologies, but they’re mostly useless since you still have to establish whether it’s murder in the first place (presumably by resorting to context and/or reasoning).
I suspect non-tautological categorical ethical imperatives are rare, and furthermore hotly disputed among ethicists. For example some groups hold that “killing is categorically wrong,” but that view is under heavy debate.
Edit: I retract my statement about non-tautological categorical ethical imperatives being rare, at least in per capita terms. Anecdotally, premarital sex and disobeying your parents would seem to be examples of things that are widely held to be categorically wrong, but certainly not universally agreed-upon.
Hear, hear! Arguably, resources like Wikipedia, the LW sequences, and SEP (heck even Google and the internet in general) are steps in that general direction.