Death being the wages of sin, Mary(note her name;innocent, chaste)planed her murders with all her wits present. Faithlessness in her best friend and husband are not high on her list of things to forgive and forget. She too is a sinner, not a saint.
Narration is interesting with details in place. The whys and wherefores are sensibly and briefly answered, perhaps due to contest rules.
"Mary wasn't sure which John saw first, the gun in her hand or Joleen's corpse ..."
It is as if killing comes to with ease.
Perhaps Mary is cool enough to surrender pleading guilty.

Engaging!

Write on!