Svidrigailov has one function in life — to satisfy his sensual desires. To do so often takes strange ways and means. He represents a type of "Ubermensch," or extraordinary man. This type feels that the world is essentially an evil place; therefore to be in tune with this universe, one must essentially be evil. Since there can be no Divine Providence whose will is stronger than man's, each individual must assert his own will and power. Since the universe is meaningless and directionless, man's main course of action is the complete gratification of the appetite. Therefore, for Svidrigailov, his pleasure and gratification are all that matter. How they are achieved is unimportant. Svidrigailov admits to Raskolnikov that he has a "natural propensity" for the vulgar. He has no scruples about getting his own way. His life has been constructed on the idea that his own feelings and pleasures are more important than anything else; therefore, he can rape a mute 15-year-old girl and, upon hearing that this girl has hanged herself, have no feelings of remorse. He simply shrugs his shoulders.
Of equal importance are Svidrigailov's acts of seeming charity. If he does perform good charitable acts, it is not because he sees the acts as good actions but simply because the impulse of the moment gives him pleasure. Likewise, in his kindness to the Marmeladov family, he is hoping to deceive Raskolnikov and Dunya into believing that he has reformed from his previous evil ways.
At last, even Svidrigailov realizes that he cannot live completely alone and isolated from the rest of humanity. When he realizes that he cannot have Dunya, he is forced to commit suicide. Suicide is the only thing left that he has not willed for himself. His old manner of living has now been denied him by his realization that he can't live alone and there is no new method left to him. Therefore, he takes his life as the only course of action open to him.