A Change of Tone in Edwards?

In comparing Jonathan Edwards' "Faithful Narrative" and his sermon, "Sinners in the Hands of an Angry God" there is an extreme change in tone. Granted one is a sermon and the other is a formal letter, but the difference in tone and writing style is, nonetheless, striking. In his account of how various peoples came to experience conversion he is optimistic that more will have similar experiences of the divine and more than pleased that these people have turned away from wickedness and accepted the divine truth of the gospels. This narrative is written four years prior to his sermon, in which he denounces all wickedness, expounds upon the torment that awaits those who do not repent and convert, and has, overall, a very negative attitude towards those who he is giving this sermon to. Was Edwards giving other sermons on par with sinners at the time that all of these people in his community converted just four years prior? Was this a typical rhetorical technique common to him and others in his time and area of New England. If not, what happened in the four years in between his narrative and his sermon that prompted him to be so unabashedly fervent and articulate in describing the horrors that await those in the afterlife, and paint of picture of the divine, not as merciful, but as malevolent and watching on with pleasure at the suffering of his creation?