And yet that's not what happens. It is mostly used for political purposes. In fact, I recall you saying precisely that some time ago.

It is a political remedy for a criminal violation. It is a mechanism by which the Congress can remove a federal officer from their office when they have committed a crime. The procedure by which Congress acts is not subject to criminal procedure, so the process of impeachment and conviction isn't a judicial criminal process. But it exists as a remedy for criminal violations. The Constitutional Convention specifically considered - and rejected - adding maladministration as an additional ground for impeachment. It is not the intent of the provision to let Congress remove a federal officer simply for doing a terrible job, but rather for depriving an actual criminal of their office.

If it was "criminal", then the end result should be prison. Not simple removal. But that isn't ever on the table. Not with Nixon, not with Clinton, not even with the Felon (who is an actual felon).

Nothing in the Constitution takes prison off the table. Congress doesn't get to send people to prison, though - only the courts can do that. If the President were to literally go shoot someone on 5th Avenue, if Congress were to remove him from office he could thereafter be prosecuted and sent to jail in due course, assuming he was convicted at trial by a jury.

I think the Founders never dreamed of things like this, so they didn't set it up that way.

I think they absolutely dreamed of things like this, which is why the Constitution has an impeachment process. Indeed, the entire structure of the Constitution is premised on the idea that whatever type of government you make, any given office you create will at one point or another be occupied by a terrible person. They were a very worldly bunch.

We also have to remember that a lot of the behaviors that we today regard as horrific violations of the public trust were perfectly ordinary practices to the Founders. These guys were 150 years before the progressive reforms of the early 1900's. There was no professional civil service, no Anti-Impoundment Act, no Administrative Procedure Act, or any of the other things that we take for granted are the hallmark of a professional civil service. Government ran on raw political power, patronage and spoils were the order of the day, and it was more likely they assumed than officeholders would be throwing the sharpest of elbows and doing political favors for each other right and left. Ironically, the fact that we've moved away from the grubbiness and featherbedding of early politics may have contributed to some of the issues we face today....but that's another discussion altogether.