Suppose you are approached by credible aliens from constellation Sadists-R-Us, and they tell you that they are planning to destroy planet Earth. However, they say, you can save it if you yourself (and they will provide you with the tools to do it): 1) round up all the children under 10 and torture them to death under excruciating conditions, or 2) infect all humans with a lethal strain of plague (but you, your friends and close relations will have immunity), or 3) roast all your close relatives or friends on the spit alive, and then eat them, or 4) insert the most hideous "unthinkable" evil you yourself can think of. Is there any point along the continuum of evil where you would take a stand and say, no, THAT I will NOT do?
To which I replied:
Let me flip your question around: consider a horrible action that you would be (understandably) reluctant to perform -- say, torturing a baby. Are there any possible consequences that would make you reconsider, that you would be willing to sacrifice your "moral purity" for? Say the aliens would torture every baby if you didn't do this one, and then they'd blow up the world as an added bonus. Is there any point along the continuum of evil where you would take a stand and say, no, THAT I will NOT allow?
Chances are, you'll feel a lot more sympathy for one or other of these two lines of attack. If the first, you exhibit symptoms of deontology, and should consult a health professional immediately for psychiatric evaluation. If the second, you have broadly consequentialist intuitions, and should not be allowed near sharp implements, babies, or political power.