For what can more partake of the mysterious than an antipathy spontaneous and profound, such as is evoked in certain exceptional mortals by the mere aspect of some other mortal however harmless he may be, if not called forth by this very harmlessness itself?
A couple of passages from the book that delve into the complexity of the antagonist:
But the thing which in eminent instances signalizes so exceptional a nature is this: though the man’s even temper and discreet bearing would seem to intimate a mind peculiarly subject to the law of reason, not the less in his heart he would seem to riot in complete exhaustion from that law, having apparently little to do with reason further than to employ it as an ambidexter implement for effecting the irrational. That is to say: Toward the accomplishment of an aim which in wantonness of malignity would seem to partake of the insane, he will direct a cool judgement sagacious and sound.
These men are true madmen, and of the most dangerous sort, for their lunacy is not continuous but occasional, evoked by some special object; it is probably secretive which is as much to say as it is self-contained, so that when, moreover, most active, it is to the average mind not distinguishable from sanity, and for the reason above suggested, that, whatever its aims may be – and the aim is never declared – the method and the outward proceeding are always perfectly rational.