Dreiser must leave Clyde's actual culpability vague in order for the 'trial section' of the novel to work.  At the end of the novel, one might claim that Clyde realizes that he is not being honest with his minister, with God, or with himself when he considers his actions on the boat with Roberta.  As he faces the electric chair and his own Judgment Day, Clyde at last realizes that his mistreatment of Roberta is what matters.  The novel ends with the Naturalist suggestion that there are no accidents; instead, there are influences, decisions, actions, and consequences.  Given the novel's unrelenting Naturalism, it is most likely Dreiser's intention that readers ultimately see Clyde's actions at the lake as morally wrong AND unavoidable.  Clyde's struggle to define his actions as accidental or planned become irrelevant while he sits in prison at the end of the novel.  In the end, Clyde's actions are all that matter -- and for these actions the novel provides the fully unhappy ending that the novel's title promises.