First of all, Doug doesn't kill the zombie teacher. Doug helps out by attacking it with a plank, but he fails to actually kill it. Larry or Mark are the ones who put down the zombie if Carley's dead.
Secondly, I fail to see how throwing away his life to save Ben was beneficial to the group. It was good of Doug, sure. But, let's face, Ben's survival didn't exactly do the group any favours.