The sum of energies and density over a whole region has to be taken into account. In particles accelerators the temperatures will be enormous at the point of collision but the densities are too low over the entire region to produce cataclymic events for the rest of us. Temperatures far higher than the internal part of the sun have been reached but the density and mass is simply not there to ignite the atmosphere.

This same paranoia existed when the comet Shoemaker-Levy was heading toward Jupiter. Each of the 13 collisions produced the equivalent energy of 250,000 nuclear weapons and gave off incredible plumes but none were enough to ignite the atmosphere. The density and mass of energies was too small.

The LHC is in the same boat. Exactly ONE cylinder of hydrogen gas (the same size of a small propane torch tank one buys at the hardware) is the total amount of fuel that the LHC will use for its particle collisions for an entire year. The amount of heavier ions will come in a smaller cylinder. You cannot bring humankind to an end with that little energy.

The amount of energy mined to create nuclear weapons was enormous. The whole reason the Soviets spied on the the United States dealt with economics and not the lack of scientific design. Zeldovich could not get a stubborn dictator to give him enough funds and equipment and personnel to produce the bomb until Hiroshima went up in smoke. All the scientists working on nuclear weapons in both countries were astrophysicists who collaborated with one another, studying neutron stars before and after the war. So each knew the other had the capability to produce the bomb. If anything was going to blow up humankind, that was it. Now we have a need for them, protecting us from inbound asteroids.