An assessment of an offender's moral culpability does not form part of the determination of the objective seriousness of their offending; they are separate but related concepts. While a mental impairment 'may' affect objective seriousness, it will not necessarily do so — the critical factors are the nature of the impairment, the nature and circumstances of the offence, and the degree of connection between them. Where an offender is rendered culpable for murder by reason of two forms of extended criminal liability (felony murder and extended joint criminal enterprise), the sentencing judge must sentence the offender for the offence they committed on the accepted basis of their criminal liability, not by reference to the principal offender's conduct. Standard non-parole periods, though not applicable to juvenile offenders, provide a useful yardstick for assessing manifest excess.
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