The leaven in the parable of the kingdom of God (referring specifically to the reference in Matthew 13:33) is anything BUT a negative usage.
To propose that the leaven here represents corrupt doctrine or heresy infiltrating Christendom to bring about the end of the age can be little more than depressing extrapolation to fit the current dispensationalist paradigm.
The direct and clear meaning of this passage is at it reads:
The kingdom of God will continue to grow and increase until the earth is filled with it, as the abundance of scripture (old an new testaments) testify.
Presented together with the parable of the mustard seed (as Christ did), the parable shows a clear hope and view of the potency and tenacity of the gospel, able to withstand and grow in all conditions, and also to make the recipient of the gospel much greater than its' original sum (the person with Christ is exponentially more than one without).