Were the disasters of the 20th century due to the triumph of secular ethics?

YES. The two world wars, purges of Lenin and Stalin, the Holocaust, the killing fields, the cold war (including the Korean and Vietnam wars) and empty consumerism were the fault of secular ethics. Only when America became more religious (starting in the 1980s with Ronald Reagan and the silent revolution) did we win the cold war and put all of this crap behind us.