Many fans have suggested that Snape's Worst Memory wasn't...
Severus Snape trembled as he raised his wand.
"Avada --" The Muggle never knew what hit her. She was dead already, Snape later found out. A reanimation charm was in effect, for Malfoy's pleasure more than anything else. Humiliating to have killed for no good reason, moreso to realise that he hadn't.
Yet, he hadn't bothered to store this in his Pensieve until now.
The memory of James Potter was much more painful, but that memory was no longer his worst, since he'd caught Harry watching.