It's an interesting philosophical question. How far are you willing to go to protect your group? How far is too far. The show tried to make it abundantly clear with the guy playing with the severed head, and the other Survivor's lovely collage, that these guys are pretty effed up. Just the fact that they were likely going to kill one of Sasha, Daryl, or Abraham just for running into them on the road tells you that no interaction with them is going to be civil and without bloodshed.
Knifing dudes as they sleep seems a bit much, but it's kind of an "Us or them" situation. They've learned the hard way that half measures usually don't work out. If they had been more proactive with The Governor they might have been able to stay at the well fortified prison indefinitely.