Wasn't the sickness just Smokey manipulating people? He got to Clair and Sayid, and he promised to give them back her baby and his girl if they did what he said. Or maybe I'm remembering wrong. But, if this is true than it is possible he did the same with the other people; telling them to kill eachother for a reward only he could give.
When the sickness infected Rosseu's team they were definately insane so its not just Smokie manipulating.