it is called supererogation.
of course
supererogation is a fragile expeditious
the ugly maw of selfishness makes it a meal delicious
a spirit of self sacrifice can seem so hard to find
maybe that's why mankind seems to be in such a bind
for example, I was in a hotel room and the little trash can was over-flowing, spilling trash onto the floor.
So I, helpful janitor that I am, began picking up the trash and compacting it so the trash can would not overflow.
My dad say "don't worry about that" and I kept right on doing what I was doing. I hate when people make messes without a care in the world just expecting that the janitors (i.e. the servants) will take care of it, and there is no need at all to make their job any easier. I said "I am not worrying about it, I am doing something about it, there's a difference."
All of which is just a long way of saying that sane, well intentioned people do not always follow suggestions with a "sir, yes sir" but often will keep doing what they think is the right thing to do.
You also seem to be unware that Zimmerman was already out of his car when he was told "we don't need you to do that".
Another time I was walking my dogs and saw a deer with its back leg caught in a barbed wire fence. First I tried to pull the wires apart and failed. Then I ran home to get a pliers and ran back to cut the fence. Nobody asked me to do that. Nobody needed me to do that, but I was trying to be a hero and save that animal's life. I thought that was the right thing to do. (Maybe I should have mentioned the baby possums that I saved, clinging to their road killed mother. When I called the county animal guy I was told "just leave them" as if babies clinging to a mother who is their food source would survive on their own (and in fact I had already left them for several hours and returned to find them still clinging to her). Or other people think that possums are disgusting animals that deserve to be eradicated. I kept them for two or three days and was about to release them when the game warden picked them up. One giuy I told the story to said "he probably took them and drowned them."
As it turned out, I was a zero. The animal already had several broken legs apparently and could only flop in the snow after I cut it loose.
In a similar way, it seems to me, Zimmerman was trying to be a hero. Trying to prevent his neighbors from being robbed. It turned out he was a zero and ended the life of a young man, but the final result does not prove that he was not sane or well intentioned.