1. A self-defense claim must be credible and consistent with the medical evidence; where an accused's explanation of how an injury occurred is contradictory, implausible and incompatible with the nature of the wound established by postmortem evidence, the defense must fail. 2. The use of a knife to stab a person in the chest with sufficient force to cause a penetrating wound piercing the heart demonstrates at minimum the realization of a real risk or possibility that such conduct may cause death, thus establishing the mens rea for murder. 3. For common purpose liability under section 196A of the Criminal Law Code, the state must prove that each accused had the requisite mens rea - either intention, knowledge that the crime would be committed, or realization of a real risk or possibility that such a crime would be committed. Mere presence or association is insufficient. 4. In sentencing youthful offenders for murder, courts should consider youthfulness as denoting immaturity and lack of foresight, warranting some leniency, while balancing this against the sanctity of life and society's interests.