The mentally ill Harlem man who allegedly butchered his girlfriend and stuffed her corpse inside a 55-gallon drum was charged with murder yesterday, but almost got off scot-free, law- enforcement sources said.

Bismark Lithgow, 24, was slapped with a second-degree murder charge in the March 2 stabbing death of Francis Angelica Alfonso Pellarano, 19.

Sources told The Post the Manhattan DA’s Office wanted to decline to prosecute for lack of evidence.

That was until the Medical Examiner’s Office got test results showing that both Lithgow’s and Pellarano’s blood was found in the West 140th Street apartment they shared, sources said.