She knows exactly what to say to wound people, and she knows that Moriarty is a sore spot for Sherlock.
He has consorted with demons, and surely the Lord will send him down to perdition.
A virtuous, happily married lady who walks out of Holmes' life the moment she recognizes him is hard to weave into a web of intrigue.
Everything goes horribly wrong at the end.
What's more impressive is Doyle showing that unconventional habits, self-determination, and a history of impressive romantic conquests — Doyle makes a point of mentioning that Irene Adler enchants just about every man in sight — are just that.
She will only keep the photograph in order to make sure the King never moves against her.