Where on a charge of murder the jury find that the person charged killed or was a party to the unlawful killing of another but are satisfied that, on the balance of probabilities, he was, at the time of such killing, an insane person whose mental abnormality was of a temporary nature and attributable solely to his own voluntary conduct in taking intoxicating liquor or drugs, he shall be convicted of manslaughter:
Provided, however, that if the jury are satisfied beyond reasonable doubt that, immediately before he took such liquor or drugs, he had the intention to kill or cause serious bodily harm to the person killed, he shall be convicted of murder.