This was never about denying true love, or tormenting souls. This was Mephisto's preemptive strike on Peter Parker and Mary Jane's child. Somehow, the child of Spider-Man would bring bad news to Mephisto, and all Mephisto's bullshit was simply a smokescreen so that Peter would never consider his kid being the target. Mephisto only revealed it after Peter made the deal, and was safe.