Andrea definitely deserved to die after taking several breaks when trying to escape zombie Milton. If she had managed to escape alive, it would have been too much "in the nick of time" to even be somewhat believable.
As far as Glenn and Maggie missing all the attackers: I believe they were just trying to scare the townspeople away and focus on the Gov and his cronies. Most of those people were running scared, and when Glenn yells "Get out of here!" he is just spraying the ground, not really trying to hit anyone.