How do you respond to the idea that Willie Stark should not be seen as immoral but instead should be seen as ex-moral, existing outside of conventional morality?
Willie Stark does not adhere to the moral code which governs the rest of the characters in the novel.
This is the most prominent and ethically questionable premise of Stark's greatness and it leads many readers to conclude that Willie Stark is, in fact, corrupt and immoral.
A preferable conclusion is to see Stark as participating in a separate moral world where the ends (maybe) really do justify the means. Willie is following a vision which shields him from judgment in a way that is not simply conceit, but which is part of his power.
Jack acknowledges this. Yet most readers do not.
Though I have to agree that Willie's moves to maintain his position and to avoid impeachment via blackmail and heavy-handed powerplay is a literal abuse of power, it seems that there is a complex moral picture here. Stark wants to remain in power to finish his public hospital, an institution which stands to do great good to the people of his state.
Willie Stark is not only keeping power for himself, but one can argue he is doing it for the good of the state.
Also, Jack Burden's great revelation at the end of the novel is that one person cannot judge another based on arbitrary codes, nor on history. Reality is sticky and complex. In the end, Jack can no longer stand in judgement on either of his fathers, nor on Anne nor on Willie.
Even if Jack's cynically driven humility doesn't make Willie a moral man, and even if nothing can, there seems to me to be significant suggestion that Willie should be viewed as an anti-hero along the lines of Kurtz and Sutpen, wrapped up in a "doom" of his own design, but great nonetheless.
The problem with this view morally is that if you excuse one person's actions by saying they exist in a separate moral universe where their actions are justified by the outcome, it is very difficult to judge others. The truth of the matter is that Willie Stark is a character who deliberately flouts moral codes and values and as a result is immoral. I don't think there is any other argument that can be used to justify his behaviour.
Part of the point of the novel seems to be that Stark is not in his own moral universe and that none of us can be, really.To think that this is possible is to invite moral corruption and the abuse of power. That, at least, is what I got out of the novel.