What is the most important fatal flaw of 'ends justify the means'?

Scott Swain asks: "What is the most important fatal flaw of '[the] ends justify the means'?"

My reply:

Without complete (omniscient) means to know what the "ends" of any action will be, before having performed the action and observe the result, there is no way to know what means must be taken which might justify themselves as allegedly legitimate "ends"; therefore if "the ends justifies the means" then no person knowing if the means selected at the time of selection, will actually bring about the ends sought, there is no way of knowing that the means selected will ultimately be justified.  Therefore this proposed ethic would make it impossible for the ethical-actor to know in advance if his chosen means will be ethical or not, because he has no certainty that his chosen action will perform the supposed/intended "end".

But perhaps more than this, if "the ends justify the means" and if all persons would therefore treat all other persons as "means" only to their own individual ends, then any act could be justified by any individual upon whatever pretenses; this would imply a negation of ethics; it would justify any means, for any ends, and therefore would imply that no unethical act could be performed, as any act may be justified upon any pretense of "ends".

"The ends justify the means" implies an ethic of sociopathy, where all persons are treated as "means" to some other's ends, while they in turn treat all others as means to their ends.... all other persons, for the individual actor, become means only, tools only, beasts only, for the accomplishment of their ends.