Catherine could have done it another way
Catherine should have instead developed a relationship w the boy and had the farmhand insinuated in his life... call him a special friend or something, and by the time the boy realizes they're lovers, he's too attached to Catherine and doesn't want her to be in trouble.
She should have known the farmhand didn't like killing people and getting him to do it a third time would ruin everything. Now she's forever alone.