"The cyanobacteria producing the oxygen caused the event which enabled the subsequent development of multicellular forms." Seems like it was ultimately a win.

And Global Cooling, as both methane and CO2 (both potent greenhouse gasses) were reduced out of the atmosphere.

The resulting glaciation extended across the entire planet, was kilometers deep, and lasted 100s of millions of years.

All because blue-green algae.

https://en.wikipedia.org/wiki/Huronian_glaciation