Complex sentence: Darnley, however, alienated many who would otherwise have been his supporters through his erratic behaviour and his insistence that he be awarded the Crown Matrimonial was still a source of marital frustration.
Split into simpler sentences: Darnley, however, alienated many who would otherwise have been his supporters through his erratic behaviour. His insistence that he be awarded the Crown Matrimonial was still a source of marital frustration.