This is merely a theory: I think Kuzan genuinely hates the World Government for their brutality, corruption and hypocrisy, and wants it destroyed. So he goes to Teach as he sees him as the one with the best possibility to do so.
But I also think there's a agreement akin to the lines of "I'll serve you, but as soon as they're taken down, that's it". And Teach agreed because he's that kind of fella.