When the Earth formed approx. 4.6 billion years ago, massive & continuous volcanic eruptions for the next billion years is what eventually created an atmosphere. And as we all know, any planet can't become habitable if it doesn't have any atmosphere.

Hence, CO2 is not all that bad 'coz it initiated a primordial atmosphere here on Earth 4.6 billion years ago. However, too much of it is what's ultimately bad just as to how it is with Venus.

*Those volcanic eruptions 4.6 billions years ago was really good for the planet 'coz it released a whole lot of water (a pre-requisite to life along an atmosphere) in the form of steam. The primordial CO2 atmosphere then helped trapped that water instead of it [water] escaping to space.