Because they tried to take their shit. Should they have handed it over and bowed to this mysterious Negan? Who's to say that they wouldn't have been killed there and then? Daryl made the right decision.
Daryl also ran into them when Dwight and his wife tried to escape. If their own people are fleeing, that tells you something.
They murdered them to stop them exploiting them. It was not a bunch of strangers that they stumbled on. It was based of their previous experiences with them and the intel from the Hilltop community. Had they no prior knowledge and stumbled across the base, and then murdered them, that would have been in cold blood.
Your logic is terrible. What do you think Rick is trying to do? Protect his people from threats. Negan is looking to exploit people and runs a far more totalitarian regime than Rick did. The wives don't actually have a choice, he wants you to think they do. In fact, Negan is leaving his people vulnerable to mutiny from other communities by not killing them. They won't forget the examples he made initially. That will be his downfall.
Negan's group massively outnumbered Daryl and co. when they met, if they wanted to kill them then they would have done so and just taken the stuff. They were given the option so that they could choose to be part of Negan's group, which is obviously the best possible solution.
Dwight, his wife and sister-in-law were fleeing because they stole ALL of the medicine from Negan so I wouldn't use that as evidence of how shitty the Saviors are. And considering how shitty a position that would have put the Saviors in had they not returned, it was pretty magnanimous of Negan not to Luicille their heads into mush.
Rick's group didn't know that Dwight and co. or the other lot that Daryl ran into were the same group who were dealing with Hilltop iirc so the 'intel' that they had at the time of deciding to murder an entire group of people was that the Saviors had killed one person and then made a deal of protection in exchange for goods. I can't see any way in which you can give Rick's group the moral high ground here, and yes it's absolutely in cold blood to kill complete strangers for material gain.
I've never denied that Rick is trying to protect his people, I just think that it's exactly the same what Negan is trying to, albeit with slightly more extreme methods which are likely down the harsh nature of the world they find themselves living in. Plus I don't think that Rick's methods are much different to Negan's, he usurped the position of power in Alexandria on the threat of violence and rules with an iron fist.
The wives absolutely have a choice, Negan made it perfectly clear that any of them could leave him at any time but that they would have to contribute to his society in other ways, as everyone else has to. You can't after all carry stragglers in the apocalypse.
If Negan faces a mutiny it will only be because Rick and co. will have coerced these poor people into starting one, just like he coerced Alexandria into his rule and Hilltop into parting from the Saviors. Hilltop were perfectly happy with their deal until they met Rick, the Kingdom are living a perfectly happy existence while dealing with the Saviors and even Alexandria were perfectly happy before meeting Rick. While we are almost certainly heading towards that war, lets not pretend it's because of anything other the fact that Rick is far too pigheaded to bow his head to a higher power even if it is for the betterment of all those around him.
Don't forget, he made Rick spaghetti as well!
Exactly, Negan is a top bloke. I bet that spaghetti was delicious.