What is Aggravating Circumstances?

Legal Definition
Factors that increases the severity or culpability of a criminal act, including, but not limited to, heinousness of the crime, lack of remorse, and prior conviction of another crime. Recognition of particular aggravating circumstances varies by jurisdiction.
Illustrative caselaw
See, e.g. Magwood v. Patterson, 130 S.Ct. 2788 (2010).
See also
Legal Definition
A condition that makes the accused more liable. The more severe punishment is given to cases with brutality. Refer to mitigating circumstance.