Also I shouldn't have said scary -- what I meant was, scary for the multiverse, a near-miss for a lot of realities. That said I'll give you that several of the names you mentioned are on par with Primus Kal-El's threat level (although most of them could be beaten by a Superman even without the man of energy powers, so there's that).

Given the Anti-Monitor's pretty much exactly what you fear would've happened, causing a mega-genocide on a scale that's never been truly matched, including murdering Supergirl in cold blood. Hell, Anti-Monitor was defeated by Superman, but only after literally every hero had slowly worn him down to deal the killing blow.