Some go into politics or public advocacy for reasons of reform, often for good reasons. They have seen flaws in education, labor laws, social practices, or a dozen other ills common to humans living together. Some of these are so obvious and egregious that they are moved to public service. Yet they quickly encounter the other reformers, and find that not all of those are of like mind. Reform movements have energy, and so attract the ambitious, the corrupt, the crusaders for other causes hoping to to steer the others. I have an impression that Causes come to be populated by worse-than-average people. Could it be that good causes are especially prone to this, or is it simply selection bias that I remember those religious enthusiasts who fell for reasons of money or sex, or the advocates for racial justice or better working conditions who found they liked the notoriety, or the power, or the joy of kicking the powerful more than the original cause.
We deal with this in different ways. Some get out of politics or advocacy altogether. These might become writers, or more intentionally religious, or one of several deep approaches to reform by dedicating to good works, founding a commune, or living out the example. They might become more moderate reformers. Some flip entirely and say that the abuses they came to fix are nowhere near as bad as what the radicals want to put in their place.
Calvin Coolidge is an excellent example. He entered politics as a reformer and behaved that way as mayor of Northampton MA. Yet gradually the other reformers, as in those supporting the police strike for political benefit without regard to the needs of the public - by their own statements, according to Cal - soured him on the group. He remained a slow reformer, according to his own lights, though his opponents disagreed. He felt they were not merely impatient but dangerous, wanting to tear down with no plan to build up.
George Orwell was an honest socialist who went to Spain. Christopher Hitchens was a great admirer of Orwell, and ended up taking a similar path, of retaining many of those principles even while finding the other socialists to be dangerous..
But most reformers and progressives continue down the original road, making accommodation with the corruption for the sake of the cause. These most often adopt a dozen causes, at first relying on the political tradeoffs to get what they want, but eventually just becoming one of the generic reformers, in favor of All Good Things. Progressives are more likely to do this, but it is common among conservatives as well. Joe Biden would be an example of this type. Bill Clinton was better able to pick and choose his causes (though he was skilled in letting people think he was on their side). I think the accusation could be fairly leveled against Bush 43, the education president who also wanted for minorities to have better access to mortgages. Those were excellent ideas that became worse than their original evils when they took the form of legislation. John McCain's Campaign Finance Reform was a terrible idea that did not accomplish what its proponents wanted, but I grant that John McCain was sincere in wanting to solve that problem. Over time he came to enjoy his role of Dissident Republican instead.