Rick is an asshole, he's not better than the guy who tried to kill him. Let's face it, if a stronger group would march into his group's home and allow them to stay in exchange for half of their food, Rick would probably do the same thing, try to take out their leader.
If the series would follow another group of characters that would run into Rick's group they could easily be seen as the villains led by a crazy ex cop who executes everyone threatening his authority. I like that, it's much more realistic than a bunch of weaklings who shouldn't have a chance in this world.
Keeping the remaining prisoners alive is a mistake and I bet at least one of them will become a serious problem before this half season is over.
If someone had brutally murdered Rick and Daryl, hunting the latter down and having him eaten by zombies, would the rest of the group just sit in their cell block thinking "I guess they won"? No, they would observe, make plans and then strike when they think the time is right, if only to protect their half of the food that the five times larger group will
come to get when they munched through their half.