In my experience McKinsey (like most consultancies) does almost all its hiring directly out of college or grad school. Hiring outside those pipelines was becoming a little more common when I left, but I'd imagine it's still probably <10% of the total.
I don't think there'd be much hiring of already 'corrupt' people, but it's pretty reasonable that when you have a very competitive and prestigious position, on average, you might find that more 'cutthroat' people are more willing to invest time and energy to getting hired.