Molly does not want to kill Estella anymore because it was jealousy and anger that first lead her to want to kill Estella. Now many years later Molly does not hold the same anger and jealousy that she did before. Molly wanted to kill Estella for the main reason that she was angry at Magwitch, which is shown from the quote "On the evening of the very night when the object of her jealousy was …
showed first 75 words of 337 total

showed last 75 words of 337 total
…or cower before her, which devastates her. Since she still cares and has feelings for Estella, Molly would most likely not want to still kill her. Molly does not want to kill Estella like she once said she was going to, because she still has feelings for her. Another important reason is because she only said she was going to do it out of spite because she was jealous and full of hate towards Magwitch.