The general design of "Srimad-Bhagavatam" is meant to prepare us to understand the tenth canto, or Krisna-carita properly. But it is not so easy to understand it due to the anarthas in our heart, they create doubts. Srila Sukadeva Gosvami helps us to overcome all these unwanted problems while describing different stories in order to understand the tenth canto.
Sixth canto plays a very special role. It tells the story of Vritrasura, and the purity of his love of Godhead is depicted in a most graphic way. Sixth canto describes a conflict between the mentality of a pure devotee and the mentality of a mixed devotee. The very strong tendency within our heart is to settle for some sort of mixture when it comes to devotion, but "Srimad-Bhagavatam" does not approve it.
Each canto of "Srimad-Bhagavatam" describes a certain clash of psychology, which is the clash within our heart. If we go deep in our heart we will see the same sort of conflict. Sixth canto describes the conflict between Visnudutas and Yamadutas, and one part of us is with Yamadutas and another with Visnudutas because subconsciously we do not have complete faith in pure devotional service. The most important and sad difference between mixed devotee and pure devotee is that mixed devotee will remain here in this world, whereas pure devotee will go back to Godhead.
Sixth canto of "Srimad-Bhagavatam" is posanam or mercy of the Lord, it explains how the Lord is merciful towards pure devotees and mixed devotees, but His mercy manifests differently. Mixed devotion brings about humiliation. The Lord reveals our attachment to the world, therefore we should try to purify our devotion. The Lord protects those who have pure devotion.