If "tone" can be seen as "the attitude an author takes towards the work," then I think that Thoreau hits a very defiant tone in Civil Disobedience. This level of defiance can be seen in how Thoreau saw his writing as a response to the clamor as to why he refused to pay taxes to a government advocating the Mexican- American War. I think that this tone is evident in how Thoreau argues the need for individual action to heed to a higher law, at times, when there is obvious conflict with human law. Thoreau is passionate about this more transcendent realm of justice and in doing so, his tone is firm and defiant in how individuals must aspire to this realm and not capitulate to what the Status Quo is. This brings out a larger issue in the writing which is that individuals must appropriate a point of view that seeks to transform and change what is into what should be. Such a task necessitates a tone of defiance and clarity in its call to action. The advocacy of resistance demands a tone of defiance to what he writes and to the manner in which it is presented. I think that this is the reason why this tone is so evident in Thoreau's work.
Additionally, the tone of Thoreau's work is persuasive, purposeful, and indignant. Thoreau argues that matters of justice should be decided by individual conscience rather than by majority consensus. He contends that all who become obsessed with the letter of the law will eventually discard common sense and conscience. Thoreau cites the example of soldiers who are given little latitude to exercise personal judgment in matters of war. These soldiers are compelled to obey, as if they are mere horses or dogs.
Thoreau's main argument is that every individual has a right to ignore "unjust laws." He is appalled that a government based on majority rule should have the authority to compel absolute obedience from individuals. Thoreau purposefully lays out his argument in support of individual rebellion in his essay. He fervently supports the motto "That government is best which governs least," and he invites all abolitionists to pull their support from the government of Massachusetts for its support of slavery.
Thoreau scoffs at the idea of "reform." He says that it's past time to talk about this. He contends that the only correct response (when one's government has overreached its authority) is to engage in outright rebellion or "civil disobedience." Thoreau maintains that "the true place for a just man is also a prison." In other words, individuals should be willing to risk imprisonment in order to bring about some substantive changes.
Thoreau resolutely argues that civil disobedience is necessary, even if it endangers the welfare of individual families and properties. He says that this is the only way to stop the government of Massachusetts from propagating slavery. Thoreau does acknowledge that it is difficult "for a man to live honestly, and at the same time comfortably." However, he purposefully encourages every individual who is serious about stopping the practice of slavery to make difficult choices. Thoreau maintains that this is the only way to live with integrity and honor.
So, we can see that the tone of Thoreau's essay is both purposeful and indignant. He is convicted of the rightness of his beliefs and firm in his support for civil disobedience.