He had to do it to protect his family. He has to think about his son and the one on the way. The men were a threat to the the farm and it resources. The more they take in the less they can provided for the the ones already there.Also they seemed to be more interested in what they have then joining the group.
rick had amazing reflexes there. if he was just a hair slower, rick would have been shot and then glenn would be shot and then the farm would be all shot to hell
the whole time i kept thinking that rick was being put in hershel's original position, and the guys were in rick's original position, and i kept wondering whether rick was gonna give in and let them in cuz he's nice like that...but then...I guess that was probably the right move.
Dave (the skinny guy) thought he was faster than Rick on the draw, and he had no reason to think otherwise with the info available to him. Gun was on the counter, his hand was close, the bottle would draw anyone else eye.
But Rick Grimes is Badass.