I've read plenty of Locke, Machiavelli, Marx, Bakunin and others but in my opinion the greatest insight on political power and human nature can be found in The Federalist Papers.
Although Federalist No. 10 gets great attention, a few minutes spent with Federalist No. 1 is a reminder of how blessed we were to have minds of that caliber influencing public policy.
"...So numerous indeed and so powerful are the causes which serve to give a false bias to the judgment, that we, upon many occasions, see wise and good men on the wrong as well as on the right side of questions of the first magnitude of society. This circumstance, if duly attended to, would furnish a lesson of moderation to those who are ever so much persuaded of their being in the right in any controversy. And a further reason for caution, in this respect, might be drawn from the reflection that we are not always sure that those who advocate the truth are influenced by purer principles than their antagonists. Ambition, avarice, personal animosity, party opposition, and many other motives not more laudable than these, are apt to operate as well upon those who support as those who oppose the right side of a question. Were there not even inducements to moderation, nothing could be more ill-judged than that intolerant spirit which has, at all times, characterized political parties. For in politics, as in religion, it is equally absurd to aim at making proselytes by fire and sword. Heresies in either can rarely be cured by persecution."