Marxism is an ideology that is responsible for millions of deaths in the last century.

Individual followers of an ideology can still be OK people. Unless you want to claim that once anyone is a Marxist, he's a bad person forever.

To bring forth their utopia, Marxists wouldn't hesitate for a second to stab us in the back.

Possibly, but we shouldn't put ourselves in a position for anyone to stab us in the back. In the meantime, we can use all the allies we can get. WSWS has done some very good work exposing this scam of a 1619 Project, e.g.