Rick killed a man in cold blood. There was no reason to leave that guy to be zombie food.
Second, Rick showed just how unstable he is by his treatment of the prisoners. If he wants to be on guard, that is fine. Just don't threaten and be such a hard . The way he talked to them and gave ultimatums was ridiculous. It was almost like he was begging for a fight. And if his goal is to protect his family, his actions were NOT helping.
Imagine if Rick had that kind of reception at the farm. .
I think this is what DiceX was talking about when he said "a line was crossed "