Morality Double Standard

[Magisterium]

Ah, the good ole 'unknown reason' defence.

Let us examine this a little bit.

Imagine that I, David, discovered a way of reading minds - even down to the intent and innermost desires. If I read someone's mind and discovered them to be completely depraved and plotting to carry out atrocities against another person, would it be morally right for me to kill them on that basis?

If you say yes , then I ask a further question: would it be morally right for me to kill them on that basis if it was within my power to prevent the atrocity through other, less drastic, means?

Morally, it would NOT be right to kill that person based upon some mystical knowledge you were able to obtain about their desires and intents. This is because the acts weren't committed yet.
In each of the cases noted, some act triggered judgement which seems harsh to the casual observer. In many cases, the most atrocious of acts are accidental (not committed willfully) and conversely, acts which may only seem minor are done with the worst of intent.

In response to your scenario (however unrealistic), Your moral obligation would be to attempt to prevent the intended acts by every other means possible before resorting to taking of life. In fact, taking of that person's life would only be morally acceptable if NO Other Alternative (IE incarceration, diversion, etc..) were possible.