Evil "by definition"

Critics of "torture" always employ the same methodology: 

i) Begin with a broad, amorphous, stimulative definition of "torture"

ii) Claim that "by definition," torture is evil

iii) Claim that various techniques of interrogation fit the definition. That makes them "torture" by definition

iv) Ergo, that makes said techniques evil by definition

But why would any reasonable person think that tendentious, circuitous method is the logical way to determine if, say, sleep deprivation is immoral? 

Why not consider sleep deprivation on its own terms? Why not consider if that's licit or illicit in its own rights? Why not consider each technique on the merits?  

Moreover, why treat various techniques in isolation to the terrorist? Has the terrorist forfeited certain prima facie protections by his actions, associations, or intentions?