Marlena was never the town pariah. Marlena was iced out of the Brady family due to breaking Roman's heart. I think that Caroline due to her own past affair with Victor, Kim and possibly Bo did not really come down on her. Marlena felt guilty for hurting and splitting her family up like any mother with a heart would. After her divorce from Roman and Belle's paternity being revealed she began to be tormented by guilt and Stefano began to visit her in her bedroom at her house at night as his "queen of the night" and that is what made her vulnerable to the devil..