Smerdyakov has to be up there with the great villains of world literature. But like Ivan's devil, Smerdyakov isn't all that intimidating a figure. He does, however, seem to have a Mt.-Everest-sized chip on his shoulder.
From a very early age, the narrator tells us, things aren't quite right with Smerdyakov. It may be all those rumors that he is the product of Fyodor Karamazov's rape of Stinking Lizaveta, the village idiot. Sure, that could give anyone a complex. But as a young boy, Smerdyakov seems naturally evil. It's not clear how someone can grow up with a "sidelong look in his eye," as the narrator describes him, but picture a shifty looking boy hanging cats and solemnly burying them and you get the young Smerdyakov – creepy and sick.
His religious education under Fyodor's servant Grigory doesn't really seem to improve him, particularly since Grigory pairs his lessons with regular beatings of the boy. You might feel sorry for Smerdyakov because he's an epileptic, but then he fakes an epileptic fit to murder Fyodor. As someone who is repeatedly described as a "eunuch," Smerdyakov positively pales in comparison to the energy and vigor of the Karamazov brothers (3.6.11) . He has neither the physical passion of Dmitri or the intellectual energy of Ivan, nor Alyosha's religious zeal by a long shot. In a brilliant touch of irony, he ends up as Fyodor's personal chef, cooking up gourmet dishes for a man (likely his father) whose voracious physical and sexual appetites he does not share.
But Smerdyakov isn't so easily dismissed. Despite his low status, he is acutely aware of the weakness of each Karamazov and plays them off each other to his advantage. Unlike Alyosha, Smerdyakov is neither direct nor truthful: he works through sly suggestions and confusing and morally ambiguous language.
Perhaps the best example of Smerdyakov's genius for evil is his masterful manipulation of Ivan. Smerdyakov confesses to the murder of Fyodor Karamazov to Ivan, but somehow convinces Ivan that he is not guilty. Either Smerdyakov is really just ignorant and intellectually inferior to Ivan – in which case he is not responsible for his actions, since he was easily influenced by Ivan – or Ivan underestimated Smerdyakov and Smerdyakov is actually his intellectual superior. Smerdyakov counts on the fact that neither option is attractive to Ivan, for it would require Ivan to admit either his guilt or to his intellectual inferiority.
Smerdyakov seems to become human only at the end of the novel, ironically, at the moment he commits suicide. "I exterminate my life by my own will and liking, so as not to blame anybody," reads his suicide note (11.10.1). "Not to blame anybody" perhaps, but Smerdyakov doesn't exonerate anybody either; his confession would have saved Dmitri from false imprisonment. Smerdyakov's final note is a kind of middle finger to the world, a rejection of his belonging at all to the common fate of humankind.
Curiously, the elder Zosima singles out suicides as those who deserve the most pity, even though it goes against religious doctrine to mourn suicides. Whether you can find it in your heart to pity the murderous, conniving, cat-hanging Smerdyakov is really a test of how you read the novel's message of love and charity.