Has Jack been infected?
Sayid and Claire both seem to have got infected in situations in which they appear to have died. I think it began for Claire at the blast in DharmaVille.
Did the blasts on the beach kill Jack? his eyes kind of opened like he had been reborn. Then Lockes sinister message ‘you’re with me now’ kind of suggested Jack had been infected with the darkness.
I think that Widmore was uncertain of the candidates motives and thats why he ordered them to be held at gun point. I think he bombed the beach because the majority of the candidates were out of harms way and as such he could kill anyone in allegience with Locke so he doesnt have an army.