He is Innocent.
Nancy asked Sid to kill her because she couldn't go through all the pain of Heroine and drugs. Sid was sent to the police station and by an educated guess he spent his last bit of money on MORE Heroine and that dosage went too far. I recommend watching 'Sid and Nancy'. The Sex Pistols and Sid and Nancy's friends said the reanactment is almost spot on. It's a mystery on what happened before Sid died though. So yeah SpringHeeLJaK is right mostly.