Gun related post here, feel free to avoid the territory

[gejohnston]

157. there is an objective standard for reasonableness

The principle of Reasonableness is really an umbrella principle that applies to each of the previous four. The issue here is whether your perceptions and conduct in self-defense were those of a reasonable and prudent person under the same or similar circumstances, and possessing the same specialized skills and knowledge (if any). If your actions were not reasonable by this standard, any claim to self-defense fails.

So, if you believed the other person was an aggressor, but a reasonable person would not have believed this, you did not act in lawful self-defense. Similarly if you believed that the threat was imminent but a reasonable person would not have, or that the force you used was proportional to the threat but a reasonable person would not have, or that you could not have avoided the threat but a reasonable person would have . . . in each case the claim to self defense fails.

It is within the contours of the principle of Reasonableness that the attacker’s prior acts and/or reputation might be made relevant at trial, even if they were unknown to you at the time. The reasonableness of your perception that the attacker’s behavior was threatening would be strengthened if your attacker had a reputation in the community for behaving in threatening manner. Similarly, the reasonableness of your perception that the attacker was acting in an irrational and frightening manner would be buttressed if your attacker habitually used intoxicants, and was in fact intoxicated at the time of the attack.

http://lawofselfdefense.com/the-five-principles-of-the-law-of-self-defense-in-a-nutshell/

In this case, Martin moved from behind the bushes in front of Jayne Surdyka's house and attacked Zimmerman. Zimmerman yelled for help for almost a minute while Martin sat on him and pounded his head in the sidewalk. That is according to forensics and two eye witnesses. That fits the definition of reasonableness as defined above.