"You can define or use it as you please, but if we want to use these terms to classify systems of magic then necromancy can be used to label any system or style utilizing death energy/working with the dead and death. Is it that hard to understand?"
No. Just... no.
No, I cannot just define it as I please. I can't redefine necromancy to mean "utilizing death energy" any more than I can redefine xSol12x to mean "stupid breadstick."
And no, you cannot use necromancy to label any system utilizing death energy, because it already has a definition. The suffix of "-mancy" comes from the Greek word mantikos , which means "prophetic." The suffix of "-mancy" itself means "divination of the kind specified by the initial element"
Thus, necromancy means divination via the dead.
Simple English and using a dictionary - Is it that hard to understand?