Alright, so there's something you've all sort of talked about but never focused on.
This is the first time Rick has killed someone...and two people. He did it without hesitation and he was prepared to do it at the beginning of the scene. Rick has never killed anyone except zombies, which he concluded were already dead by the end of the first episode so he wasn't actually killing anyone.
Broken record but again, this story is about exploring what humans would do given this extreme situation and how the zombie apocalypse would change us as people.
We're all looking for the reason Rick shot those two guys and I gotta tell you, watching it again, it's there but it's a split second. It's not obvious and given Rick's moral standing, shouldn't it be? Rick's also a cop...he's trained to defend himself but he shoots first and instead of shooting to disarm first, he shoots to kill. You can tell me that second guy, the fat one was a threat. A shot to the hand would've been easy but Rick was thinking kill from 10 minutes ago.
I think more than self defense, Rick saw a threat to the group. He saw that killing these two was for the greater good. Hell, even if they didn't attack Rick, Glen and Herschel in the bar and went on their merry way, they've learned that they have a secure farm and Rick knows they're part of a larger group. He made sure they were dead. The next scene in the show will be Herschel going nuts because Rick has killed LIVING men and Rick explaining how it was necessary.
Slight comic spoiler:
This is what has separated Rick from Shane thus far. Dale figures out that Shane killed Otis to save himself and that he belongs in this world. Shane was willing to kill to save himself and probably others. He has no attachment to Otis but would probably kill to protect others in the group. Now Rick is the same.
In the comics...if you're caught up with the latest chapter, go back and read chapter one. See how much different Rick is in the latest when he's the leader of the town compared to when he was just learning about the zombie apocalypse?
TL;DR: Rick kills a living person for the first time to protect the group. The next episode will be Herschel, Rick and probably Glen talking about how it's either wrong to kill living people (Herschel JUST came to accept killing zombies) or that it was necessary to protect the farm