@izzythepush,
The existence of corruption in any organisation doesn't mean that the entire organisation is corrupt. And the presence of honest people within a system doesn't mean the system can't have a level of corruption.
As far as I can tell (in Australia at least), the corruption tends to occur the closer to the top people get. I'm pretty sure that a decent percentage, and perhaps even the majority of those who run for office actually do it for the right reasons.
And then there are of course different levels of corruption - as different systems/cultures/environments/common beliefs etc lend themselves to differing levels of overall corruption. I think you made reference to this in your post.
In a two party political system, where one is worse than the other, some will favour the 'other major party'. I've been voting independent for many years (I don't buy the lie that 'voting that way won't make a difference' - it only doesn't make a difference if enough people buy that lie)