For good people to do bad things...

[Darwinian]

Good people use religion to rationalise the most appalling acts.Without religion,they would find other excuses (such as "the national interest")

Without religion they would not necessarily commit appalling acts as they are good. It's surely the bad people that use religion as a mask to commit bad acts and without religion it would still be the bad people doing the bad things but under a different guise.

Some very nasty things have been done by large numbers apparently good,ordinary people under atheist regimes such as Fascism, Communist Russia and the PRC.

You second point suggests the validity of my comment above as the orchestrator's of Fascism etc. where driven by their own ideology which has nothing to do with atheism per say but instead their own 'evil' aspirations.