Second, again, you use the word 'reasonable' but then you seem to think it's a personal standard; with regard to the law, it is not. The reference is not to what you think is reasonable or what I think is reasonable or even what Zimmerman thought was reasonable. The law refers to the 'reasonable person' test, which is a legal standard that attempts to be objective, not subjective.
http://en.wikipedia.org/wiki/Reasonable_person
Zimmerman doesn't get to decide what 'reasonable' means; neither do I. That is a test for the judge and/or jury, and it doesn't even mean what they think reasonable means; the standard is based on a legal fiction describing what the average person would consider reasonable. It's not objective; but it's as close as we can get.
If 'reasonable' meant something different to each person and that definition was also used in court, then the SYG laws would indeed be a license to kill for those so inclined. It means people would shoot first and later state that they reasonably felt threatened, and that would be that. But the law does not define reasonable that way, so it's not a license to kill.