It probably won't cause the destruction of Earth. That would be very ungreen. The latest word on safety was recently published in the online version of J. Phys. G: Nucl. Part. Phys. You can read about it here. But how green is it in other respects?
Well, it is costing between €3.2 to €6.4 billion to build it and run its experiments. It is the largest scientific instrument ever built. How could that money have been spent to benefit the environment?
CERN estimates that the annual power consumption for the collider will be about 800 gigawatt hours (GWh). (It could have been much higher, but the facility will not operate during the winter months.) That's about the amount of electricity used in a year by the country of Mali, or Afghanistan. At 0.5 kg CO2 per kWh, the generation of that electricity would produce 400,000 tonnes of carbon dioxide emissions. And that doesn't count the greenhouse gas emissions during construction, or from the manufacture of all the hardware, giant magnets, and so on.
I suppose it will be useful. It's just a question of priorities.