Probably Post-Partum Depression with psychotic symptoms. Either that or she is a chronic schizophrenic off her meds. Probably saw a lawyer ad telling her that her anti-psychotic meds are "dangerous" and followed the shyster's advice.
If she is a schizophrenic, she was acting under severe delusions that made perfect sense to her at the time, but to no one else. For example, she may have believed that cutting off the child's arms was the same as giving her a haircut and never expected any ill effects. Then when the child began to bleed profusely the mother became alarmed and at that point called 911. Schizophrenics suffer under delusions (false beliefs) that could potentially cause great harm to themselves or others. Some schizophrenics have mutilated themselves because of their delusions and hallucinations. Absent any other explanation I am inclined to believe that the mother is schizophrenic because that is the most probable conclusion. Since she is not a terrorist living in Fallujah, I doubt that she did it merely out of a desire to be evil. I'm no bleeding heart and I believe that she deserves punishment for what she is done, but at the same time I know something about mental illness.