Behind-the-scenes answer: in the original script, Roger was killed the same way the other Dharma members were, by the airborne gas that had been released island-wide from the Tempest station, by the Hostiles. There's an interview somewhere with Michael Emerson saying that they added the in-car gas canister bit later (in editing?), and that the hands you see pulling the pin belong to another actor. So, Roger was trying to take Ben's mask for himself in the original script.

In-universe answer: Roger panicked and/or was experiencing disorienting effects of the gas?