They always used lightning.
What was it about lightning? It wasn't a particularly effective weapon, as witness Luke's surviving it twice. It could, apparently, be deflected with a lightsaber, it didn't always kill the victim, sometimes it would restart a stopped heart and defeat the purpose. It was slow, clumsy, took an immense amount of power, and pointed straight towards the perpetrator. Smoking corpses were difficult to hide.
It did, Luke conceded with some effort, make it easier to make deaths look like an accident.
He rolled his eyes, looked at Firelord Ozai, and said, "For your next trick, thunder?"