I carry, and the situation that (apparently) happened here is something we were taught to avoid om the Concealed Carry class. If Zimmerman was standing in his own front yard, he'd probably be in his rights if Trayvon attacked him and he shot him. From what we do know, Zimmerman exceeded his authority as a citizen and as a neighborhood watch member.
When I took my last class, we'd had a nationally publicized case in the Houston area in which a man called 9-11 after seeing some guys rob his neighbor's house (the neighbor was out of town). The 9-11 operator told him to stay put and wait for the cops. The guys said "I've gotta do something". He went out with his shotgun and killed the two perps; who were both illegal POS. There was a huge outcry, just like in the Zimmerman case. The guy was never charged, and Quannell X rallied his force of occupier types to the cause.
In our Concealed class, the instructor used that case as an example of what not to do. The guy's life was not in danger if he had not confronted the perps. He told us that even though the guy was not charged, he'd probably spend all the money he thought he was going to retire on to pay lawyers. Zimmerman did not follow your dictum of "if you’re carrying, don’t intentionally put yourself in a situation where you might have to shoot someone". That's what we were taught in the class.
I believe Texas law says something to the effect, you are allowed to use deadly force if somone is stealing your property or someone elses. In that case, they had broken into his neighbor’s home and were making off with some loot. That was the reason he was not charged. Would I have done it, no but that would have been my decision.
On the use of lethal force, I believe if one feels threatened, they would be stupid to bring out anything less than a firearm. First off, if they are threatened, it is not simply a matter of someone coming to your door, it has to do with their intent to harm you. Ever been in a disabled car and have some nut come up and begin banging on the window and be within a second of breaking the glass to get at you and your family? I have...only thing that got him moving on was my revolver. I sure as hell was not going to open the window and spray mace at him from inside the car. There was no way I could take this guy physically as he was half my age and twice my weight. Message was simple deterrent, break the glass and you will be dead, period.
Most all gun fights take place in mere seconds...having the time to decide what to do puts you at a disadvantage. If you are threatened, it is far better to be in the power position than to be reacting to looking down the barrel of a gun or the blade of a knife.