A distinction of challenging behaviour and criminal offending in intellectually disabled persons is a complicated matter in forensic systems. Reinforcement contingencies and cognitive capacity are often seldom factored in behavioural incidents which are often interpreted through offence-based frameworks. This paper assessed an organized Functional Behaviour Assessment (FBA) -based decision model that was created to combine behavioural functional and recorded signs of intent with the forensic classification. The design adopted was a retrospective quantitative observational design with 60 case records of adults with mild or moderate intellectual disability. The antecedent-behaviour-consequence functional coding and the structured capacity indicators were used to analyze behavioural incidents. These incidents were mostly supported by recognizable reinforcement patterns; most of these are escape and attention functions. The use of the FBA-based framework entailed a significantly smaller criminal classification rate than that of conventional interpretation of the law. The statistical analysis showed a significant difference between classification approaches (χ² = 8.94, p = 0.003), with moderate agreement between methods (Cohen’s κ = 0.46). The results have shown that incorporation of functional behavioural analysis in the forensic decision-making has a significant impact on the result of classification. Organized use of reinforcement patterns and capacity signs can contribute to the accuracy of context and decrease inappropriate criminal labelling. The suggested framework offers an empirically based method of behavioural differentiation within people with intellectual disability and assists the validation of the same in the prospective and multi-site settings.