@Tez3 Not sure which post to quote, so just tagging you.
So I have a couple different things to try to answer your questions. Some of it will be a bit of an extension on what lklawson already said.
So first, people have a tendency to conceptualize the united states as one country. Which makes sense, since it is one country. But, IMO, when you're trying to understand either US law or US culture, that conceptualization makes it confusing. For culture, it would be better to view the different areas as their own countries, and the US as sort of a union of countries: So you have the 'new england' northeast area as a country, the tristate as another, The midwest as one, the florida up to (I'm guessing) virginia upwards and mississippi westward as another, California gets to be its own, etc. Each of those areas have vastly different cultures, prejudices against one another, and lifestyles. As a whole, there's also a difference in their moralities that effects what they care about. As a result, the laws can get pretty different.
So imagining the US as a conglomerate of a bunch of different countries, you have the union stating the bare minimum rights and laws people of each 'country' have to follow, and then within those 'countries' each state gets to decide how far or little they will extend those rights/laws. A good example is the speed limit. The US as a whole has decided on no formal maximum speed limit, however each state sets its own maximum speed limit. So the 'union' decided that this is something the states can handle on their own, no need to have the union meddle with that, and each state chose it's own speed limit. If you look at a map of the different speed limits through the US, you can see a bit of how the 'countries' impact each states decision on that.
So you can see how even though each state chooses its own, they kind of get grouped together.
And most Americans I know prefer this. The people living on long island don't really agree on a lot with the people in texas politically, and wouldn't want the same rules governing texas to govern long island (I'm pretty sure the reverse is true,
@CB Jones can probably confirm that if you want). So it extends to other things, like employment, gay marriage, legalization of marijuana, self defense, whatever. And if you travel to a different state that's not close by, it helps to treat it a bit like your going to a new country, and familiarize yourself on the basics. The good thing is, because of the union, we're still judged by the same minimum laws, and have the same rights and freedoms.
So when it gets to self-defense, it would be annoying to have to read about every state if you travel a lot. Instead, knowing the bare minimum of what you can do can help. So essentially, you can defend yourself if there's a legitimate threat to your life, and the defense will not be that you didn't commit a crime, but that you were justified in your actions. Proving that per the state is where the legal aspects will come in. It also helps to know some of the basic groupings: ie: Stand your ground vs. castle doctrine vs. duty to retreat.
(In that, green is stand your ground, orange is duty to retreat, blue treats car/vehicle with castle doctrine) So to go back to the texas vs new york idea. If you live in texas, in your mind standing your ground is okay. If you take a vacation in NYC, you should be aware that you have a duty to retreat. That's enough knowledge on that subject to worry about the rest of the consequences afterwards. If you want to take a weapon with you, check the laws of the place for a weapon, the same way you would if you wanted to travel to italy.