To call in favors he never earned for something he had no conscious control over to subvert the political process of a nation qualifies as at least a little bit dark. I think that it wasn't considered because Harry doesn't think of himself as being the one who killed the Dark Lord regularly, and he doesn't know that much about how debts in Magical Britain work. Only once he fully slipped into his Dark Side and became willing to do anything did he see that he could call in these debts.

I don't believe that Dumbledore would think of subverting the political process in that fashion. That things follow a "good process" seems to be very important to Dumbledore, even when it results in bad ends. That is the most charitable interpretation, and I believe it to be possible.