But then, is not any method which does not prolong a life equivalent to ending it?

Yes - but the distinction between doing something through action or inaction seems a very feeble one in the first place.

If unethical is just a utility cost, like you imply elsewhere, then there's still the possibility that it's ethical to kill someone to make others happier

Generally, you don't want to make any restriction total/deontological ("It's never good to do this"), or else it dominates everything else in your morality. You'd want to be able to kill someone for a large enough gain - just not to be able to do continually for slight increases in total (or average) happinesses. Killing people who don't want to die should carry a cost.