Starbuck thinks it is ludicrous to want to wreak revenge on "a dumb brute...that simply smote (Ahab) from blindest instinct". He means that the whale is not human, and in crippling Ahab did so with no malice in mind; it was just acting as a creature of its species naturally does. To be angry at him and to seek revenge seem "blashphemous" to Starbuck, an act of madness. Ahab, however, does see "insrcutable malice" in the whale's action, and he deeply despises Moby Dick. Ahab also acknowledges that he himself is "demoniac...madness maddened", and that he would "strike the sun if it insulted (him)" (Chapters 36-37).
Starbuck has another reason for protesting Ahab's intention to kill Moby Dick. He knows he will not be able to withstand Ahab's demonic obsession, and will be irrevocably drawn into involvement in his quest despite his better judgement. He can predict his captain's "impious end, but feel(s) that (he) must help him to it"; Starbuck can "see (his) miserable office, - to obey, rebelling, and worse yet, to hate with a touch of pity" (Chapter 38).