To the casual observer not familiar with the case, someone walking past her grave might mistake this as a quote from Eva Peron.
Murderers and tyrants very often feel the need to dehumanize their victims. It's an internal method of self-justification that is designed to obscure the fact that they've abused and/or killed a fellow human being.
By listing the day she 'departed this Earth' as fifteen years ago, he's trying to, in his own mind, justify every bit of neglect and abuse since that day.