Source: p 57, Criminal Law: The Basics, 1 ed (2009), by Herring
As already explained, to convict the defendant of constructive manslaughter it must be shown that the defendant committed an unlawful and dangerous act. By ‘unlawful’ the courts mean that the act must be a criminal offence. A breach of contract or a tort will not be enough. Further, a criminal offence to which the defendant has a complete defence cannot be used as the basis for a constructive manslaughter charge. It does not have to be a serious offence: theft or battery would do. However, it must require a mens rea of more than negligence. So an offence of careless driving, for example, could not form the basis of a constructive manslaughter charge. Nor, it seems, can a crime of omission.
What does the bold, NON-italicised relative clause mean? A criminal offence against which the defendant has successfully defended? Or one for which a defendant has a possible defense, but might still be guilty?
The italicised worsens my confusion, because it implies that constructive manslaughter requires a criminal offence. But then the bold, NON-italicised relative clause states the opposite?