Slit her throat, kill her parents with a shovel, and hide all the bodies in the nearest dumpster. Oh, and after killing your girlfriend, open up her stomach and abort the baby. Then eat it.
Then push the dumpster down a hill (with a natural exponential growth rate, e) towards your dad, which should surely kill him. After he's dead, take his remains and inject the blood into all the other bodies' blood.
This would eventually spawn a new strain of AIDS or hepatitis, causing an outbreak...the police and media wouldn't be focused on the murder, they would be focused on the disease. You could get away with it and move to Mexico.
Best plan ever.
Kill a man, you're a murderer, kill many, you're a conqueror, kill 'em all, and you're a god!