Both capitalism, marxism and some other ‘ism’ are claimed to be scientific. They led to various wars and killed more people than ‘religious wars’.
The “Age of Reason” brought us a lot of advances in technology.
Unfortunately, some took it too far and displaced religion with “reason” as the source of truth - basically saying that man can know better than God, so there is no need for God.
Like you said, this led to mass slaughters of people in the pursuit of societal perfection here on Earth.