You don't need to be trained in combat to know not to stand still out in the open while being fired upon. Most of us haven't been in combat and know that, and we're not nearly as generally careful as Bill. Even hunters understand the importance of cover, and Bill was presumably a very skilled hunter because he always had meat on the table. I wouldn't say that he was really in an unfamiliar position or frozen in place. His property was attacked, as it often was, and he chose to run out into the middle of the street, instead of dealing with it from the safety of the house, as usual. Yeah, unlike animals or the infected, these attackers were smart and could fire back, but that's all the more reason to be careful and stay behind cover. As soon as I saw him, I knew that he was about to get shot, because there was no other reason for the writers to make an ultra-paranoid survivalist abandon all sense and caution like that. He could've been hit as he leaned out from behind cover to take a shot, instead, but it probably wouldn't have been as dramatic of a scene, and the writers are going to tend to go with what's most dramatic over what's most believable.