If Lee had only allowed them to (spoilers)

At the end, Lee saves Ruby, which might seem to be right.

But I think it’s implied that Lee and her mom would’ve been fine afterwards if she had allowed Ruby to die

Recall that Lee during investigation discovers an algorithm. The one date missing is the 13th. We're misled to believe that Carrie Ann’s suicide fills the 13th, but it turns out to be Carter’s family

Once they get those murders on the 13th, the algorithm indicates that the killings are over, and Longlegs + Ruth’s work is done. That’s why Longlegs allows himself to get caught and killed himself: he and Ruth have just one more family to kill (ie Carter’s family on the 13th), and their deal is done. Just complete the murders on the 13th, and Ruth and Lee are free from Satan

But instead, Lee saves Ruby, which means the algorithm of killings is incomplete. As Ruth tells Lee, failure to kill the Carter family means both Ruth and Lee will burn in hell, which means Satan claims their souls. This detail would explain why the gun keeps misfiring at the end

So the question arises as to whether Lee really made the right decision to save Ruby? It’s not like she actually saves Ruby, as she isn’t able to destroy the doll that is controlling Ruby