"If we do not teach them the truth about how these terrible events in history happened"

Have you ever studied History for real? Try some Koselleck or even Paul Veyne, and you'll realise it's not about "SJW safaplace" or this north american crap you're talking about, it's something far more important it's happening in the History theory since the "linguistic turn" in 60's. Spoiler: there's no "truth" or judgement in History, only narratives and 'social forces'.
When you quoted Marx (probably without knowing it) "History repeats itself ", he said it about Napoleon III trying to get back the power in France disguised as Napoleon's sucessor. It's a "farce", it's a narrative he created to tells History.
The "dark history" or whatever you mean by that, it's not an absolute narrative, there's critical point's to review such as slavery and USA efforts to insert neoliberal dictatorship's in South american countries (such as mine for the record).
I have to agree, history is not a safe place, but neither it's a "dark spooky storyteller" book, just make a room for critical review from the known scholars such as James Lockhart, Hayden White, Kuhn and etc.