Singapore implemented legal amendments that led to a transition from a mandatory death penalty to a discretionary death penalty in some cases of murder. This has granted judges greater leeway in the sentencing of homicide offenders, with a decade having now passed since the 2012 amendment. A notable scarcity of research exists to understand the relationship between mental illnesses and criminal culpability, as well as how diminished responsibility impacts sentencing outcomes. A quantitative study of all court judgments in all homicide cases recorded on LawNet (Singapore's legal database) from 2006 to 2020 was conducted to examine the relationship between mental illness, psychiatric disputes, and court outcomes. The findings revealed that the introduction of discretionary death penalty reduced death sentences by half. The study also confirmed the presence of a mental illness is correlated to a greater sentence length, and could even act as an aggravating factor in court outcomes. This research also baselined the typologies of psychiatric disputes among homicide cases in Singapore. Further research on mitigating and aggravating factors in Singapore homicide cases and their impact on sentencing outcomes is recommended.