The problem with the plot

I know that it is a horror movie and a remake and a bit of a b movie blood and guts type of thing. I get that and I understand it. But honestly one of the major problems with it regards to plot is why did the demons change tactics. In less than an hour they possessed four out of the seven just by kissing them and screwing them. This covert part of the story would have probably netted all seven if they just stayed human. But they switch gears and they tried to kill them or shock them. If all it took was kissing or biting or screwing they would have ended the world before the first hour mark. They went totally against their intention by revealing themselves