Have you ever seen what happens in a major disaster, or even a relatively minor one in some cases? Sometimes everything is okay, but sometimes it seems like people can't wait to act like monsters to one another. A blackout in the Northeastern U.S. 10 years ago- a minor disaster- led to some decent people taking it upon themselves to do the right thing: random people started directing traffic, restaurants handed out all their food before it went bad, etc. Others tried to scam everyone else. The pizza place near my parents' apartment in Manhattan went out of business shortly after the blackout because during the crisis, they started charging people $30 for a pie.
When Hurricane Sandy hit my old house on Long Island, in a very rough neighborhood, the gun shop down the street boarded up its windows and painted "YOU LOOT, WE SHOOT" all over the place in huge letters. The reporting during Hurricane Katrina was reckless and full of nonsense, but it shows what people expect will happen during a crisis - a total breakdown of law and order.
Now imagine that the disaster isn't a storm or a relatively brief blackout. Instead, corpses are walking around devouring the living. The military can't cope. Everyone is panicking. And soon, everything has fallen apart completely, and it doesn't get better- it just keeps getting worse.
Humanity would fall back into the old patterns of fighting for survival. Most people don't know how to farm crops. Everyone is scavenging for whatever food and supplies are left laying around. The good people will try to help others, but this makes them vulnerable: at best, they will run out of food and resources pretty soon; at worst, they will be robbed and killed by bandits, or attacked by zombies before the bandits find them.
Your experience in a nice happy world don't apply here. This world is a living hell. The people at Terminus were right:
You're either the butcher or the cattle.
Sooner or later, even the good people turn bad. They have been attacked and taken advantage of too many times. They have learned that trusting people gets you killed. They don't want you to die, but they know better than to waste their time and resources on strangers who may mean to do them harm.
This didn't happen overnight, obviously, at least not in most cases, but even from the very beginning, so,e people were exploiting the crisis to their own advantage. We learned in Season 3 that Merle and Daryl initially joined the group with the intention of robbing them blind.
The sad fact is that the Merles of the world would prosper in the inevitable zombie apocalypse, and the Dales of the world would be cast by the wayside. Being good is now a bad thing. The people whose lives before the world fell apart were so awful that they were forced to fend for themselves and distrust everyone else? Those are the people who will thrive after the end. If you were already robbing and stealing and hurting others to get by before the outbreak began, you are better off than the people who still have to learn how to do those things.
The BBC made a documentary about WWII, and one episode was about the death camps. A Jewish man who had been an inmate and a guard (the Nazis selected some inmates to serve as guards, called "Sonderkommandos", and these men received extra rations and even some weapons to use on the inmates under their command) was interviewed. He told horrible stories about inmates killing one another to get an extra crust of bread or bowl of gruel. He said something that stuck with me.
I can tell you that, when humans are made to live like animals and fight every day for survival, every person on the earth, without exception, is capable of the most breathtaking cruelty and viciousness.
This is what we see happening on The Walking Dead. When being humane becomes a handicap to surviving, people cease to be humane. When being inhumane allows you to live another day, you will be inhumane. If behaving like a monster keeps Carl and Judith alive to see tomorrow, then Rick will behave like a monster.
Throughout the TWD comics, at least since the prison, Rick's group has acted only as decently as their circumstances have allowed. At the Alexandria Safe Zone, they can afford to be nice, to some extent - they have space, large numbers of people, plenty of food and water and supplies and weapons. On the road, they could not afford to be nice at all - they didn't have enough to keep themselves going for more than a day or two at any given time.
We can see examples of this dynamic in history. Men who were lost at sea in little life boats drawing straws to determine who should be killed and eaten by the others. It happens. Desperate times call for desperate measures.
But as for your observation that the real threat is other people, you are absolutely right. Everyone involved with the show and comic books says this again and again. Zombies are basically the backdrop, and are usually manageable. The far more serious problem is other living people.
