Ethics and morals vary between societies, and even within some societies. What is right or wrong can change depending on the current circumstances and the situation you find yourself in. The list below is based on current, American-style ethics and will be in descending order of “rightness” in anything other than life-and-death situations. If your life is on the line, there are no limits to what is “right” to take to sustain life. Taking more than that is frowned upon and may have consequences.
Wildlife and Wild Foods
Trapping, hunting, gathering nuts and berries, and fishing are good examples. Unless your local game warden is off his medications, harvesting food and furs from the environment around you will be the least “wrong” you can do. Private landowners may get protective of “their” wildlife, but public land and waters will are free to use until someone lays claim to them. Seasons and limits are designed to maintain the population of wildlife, but if it's “poaching” or starvation, I'd rather face the ticket for hunting out of season.
In a major catastrophe, the wildlife population may take a serious hit. Until/unless things return to something close to normal, don't expect to see as many edible animals roaming around -- you won't be the only one out there looking for food.
Trash or Debris
Dumpster diving may be an option if you're looking for materials to make a shelter or fire with. Having walked along many miles of roadways and rail lines, I can attest to the wide variety of things that people will throw out of a moving vehicle. Sorting through trash pits near campsites can often unveil gear that is still usable, if a little stained or worn.
Things Lost or Discarded
Things found along the trail are fair game. If someone ahead of you has lightened their load by tossing out duplicates or unwanted items, there is no ethical problem with picking them up and using them. As an example, fishing line and lures tend to get snagged in trees near shore and most people will just cut the line and leave them hanging. After a bit of work, you could have a way to catch fish or at least a supply of fishing line (which has multiple uses).
Possessions of the Dead
This may sound a bit macabre, but the dead have no need for things. Clothing, food, tools, shelter, weapons, etc. aren't going to do them any good and may keep you alive. As long as there are no heirs or other claimants standing around, I would have no problem sorting through a dead person's belongings to see if there was anything I could use.
However, killing someone just for their possessions is a totally different issue, one that could only be justified in an individual's own mind. There could be times where it would be justified, but unless you're dealing with a Mad Max scenario, they would be rare.
Once you get to the “Without Rule of Law” stage of TEOTWAWKI, anything not nailed down becomes the property of whoever can carry it away. Before that point, you must be prepared to suffer the consequences of stealing someone's property. Taking refuge in an deserted cabin during a storm may be acceptable to the owner, but taking his car might not. If it is a life-and-death situation, I would prefer to be alive to deal with any possible repercussions.
This is theft, pure and simple. Breaking into something for the purpose of taking what is in there is only “right” when death is the only other option. Be aware that people who lock things up are often willing to defend their possessions with deadly force.
There is at least one other category that doesn't fit into the right-or-wrong scheme:
CharitySwallowing your pride and accepting a handout may keep your belly full. It's not as common as it once was, but some people do have a sense of pride that prevents them from taking handouts. If you are such a person, offering a trade of work for food (or shelter, medicine, etc.) is the ethical thing to do.
Be wary of any “gift” that comes with strings attached (shelters that don't allow pets, etc.) because it has changed from charity to barter. If someone wants me to do or not do something in return for their “gift”, I'm going to look very closely at what else they are going to want. Refugee camps are notorious for having one-way doors, once you check in it takes a lot of work to get back out.