lacey spears of scottsville , kentucky , was found guilty last month of second-degree murder in the death of garnett-paul spears .
the 27-year-old spears force-fed heavy concentrations of sodium through the boy 's stomach tube .
judge who sentenced spears on wednesday said she suffers from a mental illness and said the crime was still ` unfathomable in its cruelty '
she showed no emotion when she was sentenced to 20 years in prison .
the boy 's father chris hill wrote on his facebook account of spears : ` crazy a ** woman needs to be beat to death in prison '