I'm not the most prolific reader of the Spectre. I read the first 8 issues of Moench & Colan series, a scattershot couple of issues of the Ostrander & Mandrake run, but I suppose I still am not clear on how the Spectre works.
He is God's spirit of vengeance and one of the powerful beings in the DC Universe. I haven't read much of these stories, but from second hand sources, an mad version of him destroyed the Rock of Eternity in Day of Vegance, turned Doctor Light into a candle in Final Crisis Revelations, and destroyed Count Vertigo's home country in the 1993 ongoing.
So why doesn't he enact old testament justice more? Why didn't he cut Hitler, Stalin and Pol Pot in half with a giant pair of scissors? Why doesn't he turn the Joker into a stack of playing cards?