Twisted? Yes. Abnormal? Hell, yes.
But if she recognizes her extreme shortcomings and does the right thing by getting intensive help or, preferably, getting the baby to a real family (and getting her tubes tied), then I'll hold back on calling her scum.
As far as the husband's refusal to cooperate, she may have to make a deal with the devil and report herself to child protection. A foster home, pending legal proceedings, is probably better than the current situation.
The problem is, we will never know the outcome. All we will ever know is what is in that letter. Based on that only, scum sure suits both of those parents.